GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: Scotchmo on December 27, 2023, 11:51:02 PM
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I’m starting preliminary work on another iteration of my Armada based big bore. For the big_bore/long_range matches that I compete in, the .257 and 7mm versions are probably near optimum for the job, and I haven’t even fully explored the 7mm’s potential as it holds together so well at 500m. But I’ve got the itch to push it further so I’ve been dreaming up what I think would be a more optimum configuration for an extreme long range, subsonic airgun. Longer distances than the current matches require.
To start, I just ordered the NOE 308-154-FN-BF2 5 cavity aluminum mold. The plan is to shoot that slug at 1080fps. It will be a stretch for this platform. The Cothran valve ports are 1/4” and could be opened up slightly, but I’ll stick with 1/4” for now. I’m somewhat limited in pressure by the Marauder/Armada air tube. It’s 3000psi but I might cheat a little and go 3100psi. Also, the tube provides only a 215cc plenum which is a little small for a planned 400fpe airgun. The barrel is going to be 60” long. I have been using DOM steel sleeved 7/8” OD barrels for the 36” long .257 and 48” long 7mm barrels. They are heavy and 60” would be more so. To keep the weight down on the 60” barrel, I would like to use a 7/8” OD, thick wall, UHM carbon fiber sleeve. That would add another $400 to an already pricey barrel. So I might start by slip sleeving the barrel in a crude steel pipe and go to a carbon fiber sleeve later on. The barrel liner is to be a TJ 1:10 twist .308 barrel that is 1/2” in diameter. According to TJ, they can make any length.
It’s going to take a little redesign of the breech to accept the longer slugs. Other changes include using 60moa of compensation in the integrated mounts. The 7mm has 50moa.
The original .257 had a floated barrel. Early testing was done with a 3D printed plastic breech. An aluminum breech was necessary to support the heavy 36” barrel. I ended up using a barrel band on the 7mm as the 48” long barrel sagged too much, even with the aluminum breech. I plan to use barrel bands on the .308 as well. Not needing the support for a steel “bull” barrel means that I might be able to live with a plastic breech for awhile.
It’ll likely be a couple of months before any progress reports.
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get a second marauder main tube , connect the 2 threaded ends and make a cap for the second tube after chopping it.. this will yeild about 400 cc and the adapter in the middle is pretty simple.. It will also offer support out front for the 60 inch barrel
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Have you considered tensioning the barrel or doing it FX style or a combination of support discs and tension? That would be much cheaper than CF and save weight over the steel sleeve. In my .257 I used two 2mm thick CF tubes one over the other to sleeve the liner as a mid priced solution.
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There are Marauder air tube extensions available (70cc extra). I may not need additional plenum.
The barrel is supported by the handguard, and is unsupported forward of that. As long as the sleeve is stiff enough, no additional support is needed. I’ve got a couple of options for the barrel sleeve. And I have considered the suggestions. I’m going with the cheapest option just to do some testing. A steel water pipe (0.85” OD, 5/8 “ ID), probably with a few -014 orings to center the 1/2” diameter barrel.
I’m not going to do a tensioned barrel. Final barrel sleeve will be either heavy DOM, UHD CF, or two layers of standard CF. The selection of 60” long CF tubing is very limited.
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I hope you don't mind if I follow along.
Mr. Hull your work is truly fascinating.
Hunter
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You can stiffen the barrel by epoxying a close fitting aluminum tube over it, less weight.
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You can stiffen the barrel by epoxying a close fitting aluminum tube over it, less weight.
Aluminum is about 1/3 the density of steel, which gives a lower weight, but it has a correspondingly lower modulus. CF tubing is a lot better.
Modulus:
aluminum = 10msi
steel = 30msi
CF = 33msi
HM CF = 57msi
UHM CF = 110msi
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Hey, Scott, always good to see what a fellow Mad Scientist comes up with.... It will be interesting to see what you can do with the 154 gr. BBT.... I have a newer design with a smaller (40%) Meplat and slightly shorter and larger BT base, if you want an alternative (3.0 cal long instead of the 2.5 cal BBT in your 7mm) maybe we can rattle Al's cage to make it?....
(https://hosting.photobucket.com/images/oo221/rsterne/New_257_cal_BBT_90_gr(2).jpg) (https://hosting.photobucket.com/images/oo221/rsterne/New_257_cal_BBT_90_gr(2).jpg)
155 gr. as a FN in .308 cal (SD = 0.233).... just scale up all the dimensions from the drawing.... ;)
I don't know if you have checked out Clearwater Composites, they sell UHM tubing in 72" lengths.... You could sleeve it in 2 stages, use regular CF for the 1/2"ID > 3/4" OD ($164), and then UHM for the outside (by far the most important for stiffness).... They go up to 72" long and up to 1" OD in UHM (7/8" OD is $189)....
https://www.clearwatercomposites.com/products/carbon-fiber-tubes/round/uhm-round/?filter_nominal-id-range=0-500-0-999
For a 60" barrel, I would go for broke (literally) with the 1" OD UHM (72" for $408).... over the SM for the .5-.75" inner layer.... That 1" OD UHM should be like a (very light) crowbar!....
Bob
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Both my .257 and 7mm barrels are 1:14 twist, which is a good match for 2.5 caliber long boat tail slugs. So if I wanted to push it, I was going to need a new barrel anyway, with a faster twist. I also wanted to stick with slugs currently in stock. I decided to redesign the breech to accept longer projectiles. Prior slugs were limited to 0.750” long and I was already at 0.710” long with the 7mm. I wanted a caliber that this platform could handle, with 30 caliber being a practical limit. I considered the 4 caliber long .257, but I think 3 calibers long is a better limit for subsonic/transonic projectiles. The 3 caliber .308 with the TJ 1:10 twist looks like a good combo to push as close as possible to sonic velocity.
Ok. Today I started machining the 0.84” pipe for the sleeve. But now I plan on skipping that idea. And this is why I like feedback on this forum. I’m going to be bold and go with a 1” sleeve. I’ve got some thinking and redesign to do.
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I made some progress on the design tonight. Since this is an Armada based design, there are some common dimensions that I retain. Because if this, the 1” sleeve will interfere with the air tube, unless I relieve it along the bottom edge. 0.946” is the maximum sleeve diameter before it touches the air tube.
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Following along. Good luck with the new rifle Scott.
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I guess I won’t be calling this latest build an “Armada”. In order to get to my design goals, I felt the need to break away from the Armada constraints. The air tube is the only Benjamin part remaining in the design layout. And it’s been modified as well as most everything else in the tube. Cothran valve, modified end cap and gauge block, and electronic hammer.
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I’ve made some progress on the design for the .308 ELR airgun. I received the slug mold and the 1” x 60” long DOM barrel sleeve. I’ve got some other materials on order. No parts remain in common with the Marauder. Though it will be fitted with a Cothran Powerhouse valve. Anyone running the Powerhouse valve over 3000psi?
Below are three images of the AutoCAD solid model.
2nd image shows just two parts. White part will be the 3D printed ABS receiver. It will be inclosed in a square steel tube that also acts as a handguard (shown in green).
3rd image shows a side view of the mostly complete assembly (minus bipod and buttstock). Finished length will be 6’-4”.
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Following, I have a similar cal. and twist barrel liner, I have been debating on a build with for some time, didn't cut it on a Daisy 880, so PCP is my next move, I had nay sayers on the twist rate, for PCP when I first got the liner ???
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Really interesting design. Is that a single shot loader that has a transfer port in it? Then you push the bullet in to barrel with that red lever and pull it back? That square steel tube and ABS receiver is certainly new. Following your build.
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now i can see why you wont need the tube extension , didnt know it was gonna get another 2l tank. ALSO yes , ive run cothran valves at 3600 , no issues aside from the common stiction on occasion.
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Following, I have a similar cal. and twist barrel liner, I have been debating on a build with for some time, didn't cut it on a Daisy 880, so PCP is my next move, I had nay sayers on the twist rate, for PCP when I first got the liner ???
I think that a 1:14 twist is about max for a practical big bore airgun. The reason one would want this faster 1:10 twist is to launch a longer/heavier/higher_BC projectile. In order to realize much benefit over the 1:14 twist, you’ll need to launch it at least nearly the same speed and preferably even faster. That takes a proportionally higher pressure or proportionally longer barrel. With this build (in it’s initial form), it will not be possible to do anything other than bench or prone.
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Really interesting design. Is that a single shot loader that has a transfer port in it? Then you push the bullet in to barrel with that red lever and pull it back? That square steel tube and ABS receiver is certainly new. Following your build.
Yes. Just as you described
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now i can see why you wont need the tube extension , didnt know it was gonna get another 2l tank. ALSO yes , ive run cothran valves at 3600 , no issues aside from the common stiction on occasion.
The Cothran valve is designed for a Marauder which has a factory recommended limit of 3000psi. I’m not sure what the limiting criteria was. Possibly for an adequate margin of safety? Did you do anything to account for that when running 3600psi? I guess 20% overfill is not outrageous, but I get leery without some justification as to safety.
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Mine was on a disco size tube where the longitudinal force was the main concern ( not hoop stress ) So upgraded hardware was needed ( I think I was using grade8 10-32 as opposed to 8-32 retention screws ) it would be different for the larger i.d marauder tube , I havent done the math as both the hoop and longitudinal forces are far greater in the mrod size tube \ Bob would be able to clarify whats needed for the mrod tube
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Really interesting design. Is that a single shot loader that has a transfer port in it? Then you push the bullet in to barrel with that red lever and pull it back? That square steel tube and ABS receiver is certainly new. Following your build.
Yes. Just as you described
What do you use to seal that loader?
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Really interesting design. Is that a single shot loader that has a transfer port in it? Then you push the bullet in to barrel with that red lever and pull it back? That square steel tube and ABS receiver is certainly new. Following your build.
Yes. Just as you described
What do you use to seal that loader?
It uses orings on the transfer ports to seal. I’m reworking that area a little bit right now. I’ll post s cross section of it later.
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After some research, I plan on using seamless 4130 tubing for the plenum. 1.25” 0.D. X 0.120” wall, rated for 3900psi working pressure. Probably somewhere between 320cc and 450cc. The thicker wall tube precludes the use of the Cothran valve. The Cothran valves have worked well in my .257 and 7mm builds. I really wanted .30” dia ports anyway, so it’s going to be a custom valve. This build now has no Marauder part interchangeability.
Two images shown below.
1st image shows the cross block (magenta/purple) positioned to seat a slug. Slug shown partially seated.
2nd image shows the cross block in the firing position.
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The block is reminiscent of a sharps, of course horizontal vs vertical. Interesting.
Dave
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If you need to build a new valve then a normal 2mm stem poppet is worth consideration. It has less friction and provides long dwell times so good for high power low fps spread.
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The block is reminiscent of a sharps, of course horizontal vs vertical. Interesting.
Dave
The Sharps has a falling block breech.
A closer match would be the Girandoni repeater airgun. Like my design, it uses a cross block. But has the transfer port behind the block, and the air charge pushes the lead ball into the barrel. Mine uses a probe to seat the slug, and the transfer port is below the cross block.
The Umarex Hammer also uses a cross block as a two slug shuttle.
I’ve also seen an interesting black powder repeater design that uses a cross block to shuttle lead balls from one side (like the Girandoni) and black powder from the other side.
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If you need to build a new valve then a normal 2mm stem poppet is worth consideration. It has less friction and provides long dwell times so good for high power low fps spread.
I’ll consider that. If I use a balanced valve with a drilled stem, it will likely be closer to 3mm as the minimum stem diameter.
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Along the lines of “go for broke”, I’ve made some design revisions. 400cc plenum, 2200psi-3900psi adjustable regulator, dual 2L bottles.
The goal is to shoot the 154gr@1080fps for 400fpe. That can be achieved with different combinations of pressure and dwell. Assuming I’m able to tailor the dwell to my liking, a good/snappy tune might be .003 seconds at 3600psi. With 4L at 4500psi, that should give 50 shots at 400fpe.
The adjustable reg can go down to 2200psi. With a .006 second dwell setting, I’ll still get 400fpe, though it will not be very efficient, and it will probably be very loud. But since that would be drawing the bottles all the way down to 2200psi, I’d get 90 shots.
Maxing out pressure and dwell (3900psi and .006 seconds) should give 14 shots at about 1350fps for 620fpe. The high BC of the 154gr slug should keep it above 1200fps out to 100yds. If I can do that, it will be an interesting study to see how it handles the drop through the transonic range at longer distances.
I’m still not sure on the valve design. I’m thinking I’ll start with a 2mm stem conventional valve. But go to a balanced valve if I can’t get the dwell that I want.
I’ve got most of the plastic parts printed already. It’s going to be a slow process for the rest of the metal parts.
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Do note that at 3600 psi the 2mm stem is likely to punch through the poppet if you use all peek poppet. I used a set screw behind the stem in my poppets to prevent that from happening.
(https://flic.kr/p/2mBFCJV)
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I think I made an error in the max fpe. It won’t be 620fpe with this slug. 620fpe was a rough calculation that did not account for the increasing air mass at that higher velocity. It would take a heavier slug to get much over 400fpe.
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The one problem with extreme barrel length is that it adds to the mass of air to accelerate.... unless you keep the dwell very short, in which case a shorter barrel may be a better choice.... Diminishing returns.... The highest relative efficiency I have seen was Marco's .224 with a 14" barrel, at 57% IIRC.... I've never exceeded about 45%.... except with very heavy slugs at ~ 850 fps.... The higher the velocity, the more air mass to drive it, relative to the bullet weight....
Bob
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.... The higher the velocity, the more air mass to drive it, relative to the bullet weight....
Bob
Unless I use a longer barrel.
If I start with a 25” barrel and close the valve at 25”, I get max fpe for that barrel and slug. If I switch to a 60” barrel and leave everything else the same (valve still closes at 25”), air usage is the same, but it doubles the fpe and doubles the air efficiency.
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Double post deleted.
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I’m doing something a little different with the scope rail. I’m starting with an inexpensive 13 slot m-lok picatinny rail:
https://www.amazon.com/gp/aw/d/B09BJ9NQ23
I 3D printed two different bases. A 60moa base for general use, and a 180moa base for longer distances that I might try.
The 60moa base will let me click out to about 550yds. I also have an additional 40moa of holdover that gets me out to 720yds.
The 180moa base will let me click from 720yds out to about 1050yds. With the additional hold under and holdover, it will cover 550yds to 1190yds.
I normally shoot at about sea level, so I’ll get a little more distance at higher elevations.
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That's a good use for a 3D printer, Scott.... I might have to borrow that idea.... Are they 100% infill, and what material did you use?.... I wonder if you could print the Picatinny Rail integral with the ramp?....
Bob
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That's a good use for a 3D printer, Scott.... I might have to borrow that idea.... Are they 100% infill, and what material did you use?.... I wonder if you could print the Picatinny Rail integral with the ramp?....
Bob
I printed out a one piece rail. ABS, 100% infill.
I had done the two piece mainly because I was not confident in holding the required tolerances on the picatinny portion. With the one piece rail, I did have to do some dressing on the part. Flat sanding the 60moa angled base since I floated the angled surface and the .3mm layers left a stair step bottom surface. I also used a file to dress the slots that I would be using.
My small Voron printer won’t quite hold this part, so I had used the large Voron that is setup with a .6mm nozzle and .3mm layers. Tolerances can’t be too tight, especially with 100% infill. Any variation in filament diameter also affects it. And warping can be a problem.
PLA is better in almost every case, except it has too low of a plastic transition temperature, so not good in hot environments. Though I have used it for parts (including a gun stock, that have been in use for four years). Just be careful with temperature exposure.
Next order of filament will likely include ASA, which some are finding as a better alternative to ABS.
A couple of pictures are shown below comparing the two piece to the one piece. Either will probably work fine.
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I’ve printed all the ABS plastic parts. Most of them are 100% infill. I read that acetone treatment can increase bonding in the 3D prints. So these were treated by brushing with pure acetone. That washed out some of the black dye and left a blotchy gray surface. Final treatment was a 50/50 mixture of acetone and Rite synthetic “charcoal” color dye to restore the black color.
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Nice looking parts, Scott.... Is there any part of the receiver subject to air pressure?.... or are those just mock-ups to check the design?....
Bob
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Nice looking parts, Scott.... Is there any part of the receiver subject to air pressure?.... or are those just mock-ups to check the design?....
Bob
The cross block contains the 90 degree transfer path, so it holds momentary pressure. It is ABS, and I’ll probably test it that way. The plan is to replace it with an aluminum version.
All other pressurized pieces that go inside the receiver are aluminum/brass/steel.
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OK, I understand the design now (I think).... The green parts carrying the O-rings in the drawings in Reply #22 are metal, then?.... and designed with usual safety margin without the support of the cross block?.... I am nervous about printed parts delaminating, the layer-to-layer adhesion results in Z-direction tensile strength of only about 3000-6000 psi (X-Y about twice that), depending on material and layer thickness and width.... Check out my thread on Tensile Testing in the 3D printing section....
Bob
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OK, I understand the design now (I think).... The green parts carrying the O-rings in the drawings in Reply #22 are metal, then?.... and designed with usual safety margin without the support of the cross block?.... I am nervous about printed parts delaminating, the layer-to-layer adhesion results in Z-direction tensile strength of only about 3000-6000 psi (X-Y about twice that), depending on material and layer thickness and width.... Check out my thread on Tensile Testing in the 3D printing section....
Bob
The green cylinders are horizontal and vertical transfer port tubes. They will be brass or steel. I might eliminate the upper o-ring from the vertical transfer port, as that o-ring is constrained by the plastic receiver. Though not holding air directly, the receiver sees force when the o-ring is under momentary pressure. I used o-rings constrained in a plastic cross block in an earlier build and it was not an issue.
I’m also considering eliminating the horizontal transfer port tube as my last two builds simply used a single o-ring sealing against the barrel face. I added the horizontal tubular port this time to theoretically get a more positive seal as the back o-ring provides an energized seal of the tube against the barrel, in a similar manner as the vertical transfer port with its lower o-ring pushing the transfer port tube up against the cross block. The barrel can be pushed in tighter against the cross block to get a good seal if I decide against the more complicated energized seal. That worked fine in the past though it required a careful setting of the barrel pushed against the cross block before securing the barrel position in the receiver.
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I received an important part today. The TJ barrel liner is 61.25” long x 0.498” diameter. It’s a .308/1:10 twist. So far TJ has been able to make any length barrel that I’ve asked for. They are in Kentucky, so maybe they make long liners for Kentucky long rifles. The liner will go into a 60” long steel DOM tube 1” OD x 0.5” ID.
Now I just need to get into the shop more often to make some progress.
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Scott, you inspired me to 3D print tank clamps for my two benchrest rifles to attach the rear of the tanks to the plenum tube....
(https://hosting.photobucket.com/images/oo221/rsterne/3D_Printed_Tank_Clamp.JPG) (https://hosting.photobucket.com/images/oo221/rsterne/3D_Printed_Tank_Clamp.JPG)
Three 6-32 x 1.25" SHCSs at different heights tighten the clamp in place....
(https://hosting.photobucket.com/images/oo221/rsterne/Clamp_Fitted.JPG) (https://hosting.photobucket.com/images/oo221/rsterne/Clamp_Fitted.JPG)
Looks a lot better than just using a hose clamp and a spacer block like the one in the center of these clamps....
Bob
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Very nice Bob. I decided to hold off on my dual bottle version for now. It complicates the plumbing so more to deal with. Single bottle for now. Got my 400cc 4130 plenum, tank, regulator, scope, and a few fittings. I made some design changes and had to reprint most parts. 3rd times a charm so this last one should work. I had switched filaments and didn’t realize I was now using Sunlu “EasyABS” instead of regular ABS. Temperature and other settings were way off. This stuff is in between PLA and ABS. Doesn’t warp like normal ABS and runs at about 230 C nozzle temp. No ABS smell that I could detect. Layer-to-layer adhesion is super good. And so far haven’t had any moisture absorption problems. It should work better than regular ABS with unenclosed printers. Not sure if I want to compromise on the temperature resistance in the future, but I’ll print my one other roll of it before deciding.
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The plumbing problem goes away, I would think, if you put the tank block and regulator at the front like I did.... The only complicated bit is the block itself.... The tanks feed the regulator in an inverted "V" arrangement, and the reg. feeds the plenum at the front....
(https://hosting.photobucket.com/images/oo221/rsterne/Regulator_Assembly_and_Adapter.jpg) (https://hosting.photobucket.com/images/oo221/rsterne/Regulator_Assembly_and_Adapter.jpg) (https://hosting.photobucket.com/images/oo221/rsterne/Tank_Blocks.jpg) (https://hosting.photobucket.com/images/oo221/rsterne/Tank_Blocks.jpg)
(https://hosting.photobucket.com/images/oo221/rsterne/700_cc_Assembly.jpg) (https://hosting.photobucket.com/images/oo221/rsterne/700_cc_Assembly.jpg)
Bob
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.... The only complicated bit is the block itself....
Bob
Exactly. That’s the part I decided to forego for now.
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I made the 400cc plenum today. It’s a 4130 seamless tube. 1.25” O.D., 0.120” wall thickness, 36” long.
I contemplated using socket head cap screws to hold the end cap and valve, but I prefer threaded plugs. I would have tried to single point the threads, but my lathe has only a 7/8” bore in the headstock spindle. In the past, I have single pointed internal threads in ~18” long 1.25” tubing by using the steady rest, but the 36” tube is too long to do that.
I found a 1-1/16-20 tap on EBay for $30.69. Then I had to come up with a way to tap a straight hole. I used blue tape on the tap to mark the 3/4” depth. I started with the tap in the three jaw chuck and the tube in the steady rest. I was able to start the tap fairly straight for about one turn, but had no good way to grip the tube to turn it. It takes a lot of torque to turn such a large tap. I flipped it around with the tap still engaged and locked the headstock. I used two open end wrenches to carefully turn the tap, and clear chips, checking often to make sure it was still going in straight. The threads came out good.
At 4000psi internal pressure, this size 4130 tube has a 3.5 to 1 factor of safety using tensile yield strength of 63ksi. I plan on running at about 3600psi, but might go a little higher.
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VERY interesting design!!!
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Years ago, when these taps were more expensive, I had made my own to thread my home built airgun receivers for AR buffer tubes/buttstocks. My year 2000 version of AutoCAD can’t easily model threads, so I used that tap to thread the ABS .308 receiver. I did a trial assembly of the major components today. I still need to make the valve, electronics, etc.
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looks incredible Scott , would be cool if the next one was dressed as a kentucky long rifle 8)
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I finished the electronics today. The 8.xx volt readout on the left is the battery voltage. The other readout is the capacitor charge voltage. The capacitor voltage can be adjusted up to about 60v to 62v. That is one of two hammer strike adjustments. The other is a plunger travel adjustment screw. When the trigger is pulled, an SCR is latched on and dumps the capacitor into the solenoid. A plus for using an electronic hammer is that the trigger mechanism can be a simple switch that makes for an easy to design/build trigger group with a good feel.
I ordered some 4140 steel for the valve and end cap. 1.25” round bar stock. Also an 8 ounce bottle of NU-BLUE to try on the steel parts. Up to now, I’ve always used Super-Blue.
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The 4140 steel bar is scheduled to arrive tomorrow. I plan to start on the valve build later this week.
I cast a bunch of the NOE 154gr (actually drops at 152gr) BBT and they came out OK. A little more wrinkling than I get with smaller 7mm cast slugs, but mostly acceptable. I really want to get the 172gr BBT if either NOE or Arsenal decides to make the mold. The 172gr might be more suitable for this build with its 1:10 twist barrel. I ran some performance numbers again and these are the predictions that I got:
Regulated to 3200psi, with .0032 second dwell, 152gr@1100fps, 411fpe, >30 shots from a 4500psi fill.
Regulated to 3600psi, with .0032 second dwell, 172gr@1100fps, 462fpe, >20 shots from a 4500psi fill
The longest/heaviest slug that will fit in the breech would be about 210gr. If I go for near max, I predict this:
Regulated to 4000psi, with .005 second dwell, 210gr@1225fps, 700fpe, 6 shots from a 4500psi fill.
All seems like a lot to expect, but I’ll be happy with 400fpe.
Here are some pics showing the rifle assembled with most of the finished/blued steel parts. Shown next to my 7mm build.
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What a BEAST!!!!
Bob
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looks awesome , I like the box tube breech
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That is TOO COOL!!
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I’ve been making some good progress on the valve body. But slow going machining chromoly steel. Being careful not to make mistakes that would cause me to start over. Very careful when tapping a couple of deep 6-32 holes. I’m an amateur machinist at best.
The numbers say that it needed to be a balanced valve, so that’s what it will be from the start. I using my same electronic controller design. Some of the valve specs:
0.079” (2mm) stem
0.332” throat
0.313” ports
0.09” lift
0.281” balance piston
About 0.013” valve seat overlap
I want it easy to open, but also want a good seal. The balance piston will keep the load on the seat low enough, so I’m going to start with an acetal valve head.
Cross section of model shown below, minus the two return springs (one for valve, one for solenoid). And a picture of just the main valve parts exploded.
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Good design, glad to see you have the O-ring on the balance piston loaded the right way to avoid stiction.... Think about a radius on the front inner corner of the valve port, that is more important than the one on the outside of the corner.... If you can, angle the vertical part of the port about 20-30 deg. towards the seat, making the corner much more gradual.... You might have to move the seat forward a bit (or the transfer port back), if it's not too late for that....
Bob
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Good design, glad to see you have the O-ring on the balance piston loaded the right way to avoid stiction.... Think about a radius on the front inner corner of the valve port, that is more important than the one on the outside of the corner.... If you can, angle the vertical part of the port about 20-30 deg. towards the seat, making the corner much more gradual.... You might have to move the seat forward a bit (or the transfer port back), if it's not too late for that....
Bob
Too late. I finished machining the valve body today.
I also started the internal valve parts today. Transfer port, balance chamber, balance piston, and grate, will be brass for this first try. Factor of safety is not great, but a failure is not catastrophic. Turning brass parts is so much easier.
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[/quote]Good design, glad to see you have the O-ring on the balance piston loaded the right way to avoid stiction.... Think about a radius on the front inner corner of the valve port, that is more important than the one on the outside of the corner.... If you can, angle the vertical part of the port about 20-30 deg. towards the seat, making the corner much more gradual.... You might have to move the seat forward a bit (or the transfer port back), if it's not too late for that....
Bob
Too late. I finished machining the valve body today.
I also started the internal valve parts today. Transfer port, balance chamber, balance piston, and grate, will be brass for this first try. Factor of safety is not great, but a failure is not catastrophic. Turning brass parts is so much easier.
Making that inner corner of the port rounded netted me 40 fps in my Evanix ...
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I got a little more work done. I still need to drill the vent holes for the pressure assisted valve (I don’t really like the term “balanced valve” in this case). I’m not currently setup to hold and drill tiny #60 holes in any precise way. I have some fixturing to do. I have a dozen #60 bits coming and a chuck that will let me hold and drill in my press and lathe with bits down to #83 (0.012”).
I’m leaving for Oregon in couple of days. I’m going to a three day airgun match that will have a long range event, out to 300yds. I’ll be shooting my 7mm in that event. The small drill parts should have arrived at home by the time I get back from Oregon, so work will resume then. There is a chance that I might have the .308 sorted and ready for RMAC (less than two months to go).
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I've been making progress on my .308 build. Hopefully, initial range testing starts on Monday.
Yesterday, I pressurized the air tube/valve assembly to 3600psi. Dunked it in water. Bubbles came out of the transfer port, when filling. Almost stops bubbling at 3600psi, but still a few bubbles. I’m not going to worry about it for now. It might seal when I fire it a few times. It’s an acetal (like Delrin) valve head.
Yesterday, I attempted a full assembly of all parts and discovered some issues. Some tolerance stack ups prevented full assembly. Also, there were some difficult access points. I made some design corrections and completed them in the shop today, along with a new slug sizing die. The first .308 die was too short and could not reduce the nose quite enough, so seating of the slug was very difficult.
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Everything went together smoothly tonight. New plans are to start range testing tomorrow.
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I got off a few shots today. On the 60v setting, I was able to open the valve all the way up to 3600psi which was as high as I got before the plastic receiver let go. The steel housing held any plastic shrapnel in place. The max velocity that I saw was 839fps (238fpe). But shots were accompanied by blow-by out of the breech block as the plastic barrel clamping was not up to the task of holding the barrel and shroud in place axially. Design mods will include a steel or aluminum barrel clamp tied to the steel enclosure rather than the plastic receiver. I’m going for at least 400fpe, so 238fpe is too low. There was significant blow-by, so that could have been one reason, but the dwell also seemed short. I’ll deal with that after I get the barrel secured better.
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Finally had a successful day at the range with the .308. Yesterday was cut short. I had mistakenly used a 40v flyback diode and it let the smoke out after I turned the voltage to a little over 60v. I fixed that and went back today. No luck controlling the dwell electronically. This initial balanced valve configuration has just about enough dwell to reach my minimum goal of 400fpe. It was able to get the 152gr to just over 1080fps. But close to valve lock. I settled on 1060fps. 5 shot groups at 100m were a little over 2moa in light winds. Next trip to the range will be for 300m testing.
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I did a few valve mods in an attempt to increase the valve dwell. I got off a few shots before the valve failed in a mode that I had not anticipated. I have a step on the balance piston to keep the hammer from pushing the piston forward through the valve head. But instead, the piston got pushed backward in the valve head by closing pressure. I had also used super-glue to hold it in place and that sorta worked as it sheared off a little bit of Delrin when it shifted. Next iteration will have the balance piston and valve head threaded together to resist axial forces in both directions. I also plan on switching from Delrin to PEEK.
I did reach a milestone as the last 4 shots were above my minimum design goal of 400fpe. The 1122fps shot being at 420fpe.
These few shots are not conclusive but it did give me enough data to start to analyze the "Mach trimming" effect at these velocities.
The highest velocity shot was right at Mach 1. I compared that shot to the next two slightly lower velocity shots. There was almost no Mach trimming between the Mach 0.97 and Mach 0.95 shots. But the Mach 1 shot definitely scrubbed velocity at a higher rate. There was significant trimming between the Mach 1 and Mach 0.97 shots. The velocity spread at the muzzle was 32fps, but was almost half that by 30yds and only 10fps after traveling 100 yards. That reduction in velocity spread will show up as significantly reduced vertical dispersion at farther distances 200yds to 600yds. One series of shots is hardly definitive. I'll be looking into it more later.
I'm going to take a break from the .308 development for a few weeks as I need to prepare for the 2024 RMAC match. The .308 is not at all ready for that match, so I'll be shooting my 7mm again. I took 1st in the RMAC Magnum Slug match last year with the 7mm and some luck. The competition looks just as tough this year.
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I’m working on the .308 again. I got about 30 shots off this time before the grate that supports the balance chamber let go. After I had made it, I realized I had selected too large of a drill bit for the holes in the grate. I still thought I might get away with it but it was too weak. There’s a lot of violent action going on during a shot cycle. Picture shows a grate with correct hole size and also shows the broken grate.
Today’s testing was done tethered, while I tried different plenum pressures. I might do more experimenting later at higher pressures, but after today’s tests, I’ve decided to go with a 3200psi regulator set point. That puts me about where I wanted to be for this build.
I tested both the 151gr and 171gr Arsenal BBT slugs. The BC of both slugs is so high that I’m thinking the lighter slug with the higher velocity and more sedate (less recoil) shot cycle will be my choice. The 171gr probably has no appreciable advantage until past 700yds. And I’ve never shot that far.
151gr .308 Arsenal BBT
3000psi 1054fps (372fpe)
3200psi 1080fps (391fpe)
3500psi 1106fps (410fpe)
171gr .308 Arsenal BBT
3200psi 1051fps (419fpe)
3250psi 1055fps (423fpe)
3300psi 1064fps (430fpe)
3400psi 1070fps (435fpe)
3500psi 1076fps (440fpe)
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Yep, there is a lot of force on the cage/grate/spider that supports the balance chamber, I shattered one and had to build it stronger.... The total area of the 6 holes in mine is double the throat area, so I don't think the valve is starving for air on the inlet side.... Making it thicker helps as well.... The load is not just a static one, the poppet can hit it like a hammer, either driven by the hammer (a real no-no), or just coasting when opening rapidly.... I use an O-ring to help cushion the blow and decelerate the poppet more slowly instead of allowing it to crash into the spider....
Bob
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I float the piston with enough room it doesn’t hit anything. My cage sits far enough forward to give the system room to cycle without crashing.
Looks like the project is progressing nicely Scott.
Dave
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I float the piston with enough room it doesn’t hit anything. My cage sits far enough forward to give the system room to cycle without crashing.
Looks like the project is progressing nicely Scott.
Dave
What do you mean by “float the piston”. The piston needs to hit a stop after opening.
I have two grate/cylinder configurations that I have been testing. The first has very little volume remaining when the piston bottoms out. So the “crash” should be a low force impact. It also has a “spherical” connection at the axial grate connection to insure alignment with the piston. A screw head holds the front and a step holds the rear. That’s the configuration that broke the grate. There is over 200lbs of pull as cycling force in the cylinder. It’s possible that’s what broke it rather than the “crashing”.
The alternate grate/cylinder configuration uses a retaining ring to hold the cylinder axially in the grate with a slightly loose connection fit to aid alignment with the piston. It has a larger volume (done to increase dwell) remaining, so the “crashing” force is higher. It was originally bottoming on the Delrin poppit, and deforming the Delrin. I’m going to make a tubular insert to go in the cylinder so that the piston bottoms out on that.
I’m also considering putting an o-ring cushion inside the cylinders.
Bob Sterne did some analysis on the Labradar data and came up with some BC info on the two slugs that I’m using.
151gr
G1=0.35
RA4=0.26
G7=0.15
169gr
G1=0.42
RA4=0.32
G7=0.18
The G1 is what most airgun slug manufacturers use as it gives the biggest number so it’s used for “bragging rights”, however, it is a poor fit for high BC slugs in the transonic (>900fps) range. I have been using RA4 and it allows for better predictions at high velocity and long distance. The latest slugs are 3 and 3.5 calibers long. That long shape is getting closer to the G7 profile, so I will be checking to see if that would be a more appropriate drag model.
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I just noticed that your cylinder/piston are reversed of mine. In yours I would float (maybe not the best terminology) the cylinder. Of course the piston (poppet) will hit the cylinder wherever you design it to, but once open, there is no reason to limit it by having the assembly limited by the cage. The cage, imo, should only serve the purpose of holding the cylinder (piston in my case) and piston apart when valve is closed. My design also allows adjustment of chamber volume without changing parts.
I’m not trying to say it’s the best way to design one but I haven’t had a cage issue and I can tune relatively easy.
Dave
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The piston needs to hit a stop after opening.
I'm not sure that is necessarily the case, Scott.... Valves do not usually open more than about 1/4-1/3 of the throat diameter, so for a 0.300" throat ID, the poppet probably does not open more than 0.075-0.100".... The exception would be if the valve is still open when the slug exits the muzzle (or nearly so).... If it closing anywhere around mid-barrel, I doubt it is opening more than 0.100".... If the stem projects that far behind the back of the valve, then it cannot be driven open more than that.... If you have, say, 0.150" before the piston crashes into the spider, it will only be doing so with it's own momentum, not driven by the hammer.... An O-ring cushion between the two, to prevent it slowing "instantly" on impact with the spider, will greatly reduce the peak impact force....
How are you measuring the dwell?.... Have you ever tried measuring the lift?.... It can often be done with a simple hammer follower with an O-ring sliding on it....
Bob
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The piston needs to hit a stop after opening.
I'm not sure that is necessarily the case, Scott.... Valves do not usually open more than about 1/4-1/3 of the throat diameter, so for a 0.300" throat ID, the poppet probably does not open more than 0.075-0.100".... The exception would be if the valve is still open when the slug exits the muzzle (or nearly so).... If it closing anywhere around mid-barrel, I doubt it is opening more than 0.100".... If the stem projects that far behind the back of the valve, then it cannot be driven open more than that.... If you have, say, 0.150" before the piston crashes into the spider, it will only be doing so with it's own momentum, not driven by the hammer.... An O-ring cushion between the two, to prevent it slowing "instantly" on impact with the spider, will greatly reduce the peak impact force....
How are you measuring the dwell?.... Have you ever tried measuring the lift?.... It can often be done with a simple hammer follower with an O-ring sliding on it....
Bob
Dwell is calculated as what’s needed for a given velocity/fpe. Though I now think the actual dwell is considerably more, especially after today’s tests.
I had decided to go with 3200psi, so I adjusted the bottle reg and ran today’s tests from the regulated bottle. The latest valve configuration had the large balance chamber and additional valve lift.
151gr .308 Arsenal BBT
3200psi 1098fps (404fpe)
169gr .308 Arsenal BBT
3200psi 1078fps (436fpe)
I’m likely running at the max FPE for 3200psi. I think that the dwell is too much, as the muzzle blast and recoil was significantly greater. I only got off 12 shots before the valve let go and stuck full open. Velocity was the same when it stuck open, but the muzzle blast and recoil was horrific. Almost got a scope eye.
The valve stem protrudes only 0.098”, so best case, that’s as much lift as the solenoid can provide. However, the balance piston pulls the valve open almost 0.3”. With small lift on a big poppit, with a lot of air flow, there is a strong “suction” attempting to close the valve. The excess lift is done to reduce the “suction” at the poppit face, and allow the balance piston to hold the valve open a little bit easier/longer.
But as I stated, dwell is probably too long now. I’ll be cutting it back a little until I see 1080fps and 1050fps for the 151gr and 169gr slugs.
The valve failure this time was a circlip coming loose along with the valve stuck full open. The grate was fracturing but had not quite let go.
The plan now it to make a one piece grate/balance chamber out of chromoly steel, and try again. Also time to try a peek poppit.
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Well, you certainly have a very hard crash happening!.... The cylinder appears bent, and the poppet is hitting the end of it!.... Is there somewhere you can install a bumper to slow the deceleration on impact?....
Bob
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Well, you certainly have a very hard crash happening!.... The cylinder appears bent, and the poppet is hitting the end of it!.... Is there somewhere you can install a bumper to slow the deceleration on impact?....
Bob
Best I can guess, it’s got to be the inertia of the piston as it strikes the cylinder. I even had a Delrin spacer cylinder inside the brass cylinder, to act as a stop. But the Delrin was smashed farther into the cylinder bottom, squishing past the step. The center of the grate was pushed back like a diaphragm.
Besides the initial plunger strike on the stem, there is also about 200lbs of force accelerating it forward.
There is at least a few fpe of energy in the moving piston as it hits the stop.
Besides switching to chromoly steel, I might reduce the balance chamber volume and incorporate a damper. Or maybe try keeping the chamber volume but limiting the travel to 0.1” to reduce the piston energy.
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A crude drawing to help explain what I meant.
Dave
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Yes, Dave, I liked that design and borrowed it for my dual tank benchrest....
(https://hosting.photobucket.com/images/oo221/rsterne/Reversed_Poppet_GA.jpg) (https://hosting.photobucket.com/images/oo221/rsterne/Reversed_Poppet_GA.jpg)
I am building 2 versions, one with an all PEEK poppet (2400 psi), and one with PEEK seal and aluminum cylinder....
(https://hosting.photobucket.com/images/oo221/rsterne/Balance_Chamber_Parts.JPG) (https://hosting.photobucket.com/images/oo221/rsterne/Balance_Chamber_Parts.JPG)
I had a Spider failure in the first valve in my 6mm....
(https://hosting.photobucket.com/albums/oo221/rsterne/6mm%20Sporter/.highres/Broken%20Valve_zpsa3kdn2zs.jpg) (https://hosting.photobucket.com/albums/oo221/rsterne/6mm%20Sporter/.highres/Broken%20Valve_zpsa3kdn2zs.jpg)
It was replaced with a much beefier Spider with just 8 holes, and over twice as thick....
(https://hosting.photobucket.com/images/oo221/rsterne/New_Valve.jpg) (https://hosting.photobucket.com/images/oo221/rsterne/New_Valve.jpg)
The valve at the top of this post is essentially this one with the piston and cylinder positions reversed....
Bob
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Scott, the problem in having the piston slamming into a solid metal stop is that the impact is many times greater than the force accelerating the piston.... I'll try and give a simple example for momentum of piston = 1 FPE....
Maximum force when hitting a stop is therefore the energy divided by the distance travelled during deceleration.... That is an exponential curve, and if the stopping distance is zero, the force becomes infinite.... Distance in feet is inches/12....
(https://hosting.photobucket.com/images/oo221/rsterne/Force_on_Impact.jpg) (https://hosting.photobucket.com/images/oo221/rsterne/Force_on_Impact.jpg)
You can see that once you are asking the piston to stop in less than 0.010", the force goes through the roof.... It doubles if you ask it to stop in 0.005".... A simple 0.070" CS O-ring as a stop can easily compress 0.010" (or more), and keep the peak force within reasonable limits....
Bob
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Scott, the problem in having the piston slamming into a solid metal stop is that the impact is many times greater than the force accelerating the piston....
Bob
I know that. I’ll look into “floating” it.
Thanks David and Bob.
I had the piston floating on a rubber oring on the first version that failed. That might be enough if I reduce the piston travel.
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If you do decide to try one Scott, I think you will like it. It just adds some versatility/options by taking the crash limitation out. I’m sure you realize but it can tuned to be more hammer dependent or more blow open dependent. As such, you don’t have to limit your hammer travel (unless you want to for tuning). I think also by taking the crash away, it takes that addition to harmonics out but that’s only speculation.
Good luck,
Dave
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I decided to try something with the original valve configuration before I did any major redesign. The poppit front was chewed up a little from the last couple of failures but the Delrin face was still good and still secured to the piston. I made a new grate and assembled everything again with three x-rings (square section o-rings) inside the cylinder to limit travel/lift to about .08” and act as a cushion. I went to the range today and tested for quite awhile, maybe 80 shots with no failures. Velocity was down a little as expected, and the shot cycle was sedate.
151gr at 3200psi
1075fps average
169gr at 3200psi
1020fps average
I took it back apart after I got home and everything looked good. I reassembled it with just two x-rings. That will give about .15” of lift and a little less cushion. I’ll retest tomorrow.
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I haven't peeked in much lately, life and work has been too busy for the hobby this year... This thread and build were a nice treat, as always thank you for sharing.
The spider/piston carrier does not have to be held captive in the direction opposite the valve seat. On my Bulldog's crude simple balanced valve the spider simply has an outer oring which prevents it from moving forward prior to filling. Once filled closing forces against the piston keeps the spider firmly in place against the bulldog's tube adapter/valve carrier.
The chamber height adjustments shared by Bob, Dave, Matt, and others are a game changer for tuning dwell, IMO. I start tuning with max chamber height/volume and a tiny vent--.015 or so, as a means of finding my upper limit of energy, If I want to make less power than max, I slowly increase jet size until finding my desired upper limit. Once that upper limit is achieved, I will decrease chamber height until achieving the desired shot cycle. It can be very snappy, or it can be very Cothran-like. Then finish the tune with whatever hammer adjustments are available in the platform.
the stem and piston are Cothran. But the stem vent has been increased to .060 or larger. I thread the top of the stem 6/32 and make jets from 6/32 setscrews. Moving the jet/vent to the very base of the chamber was key to making the valve more responsive to chamber height and hammer spring, IMO.
(https://i.imgur.com/ndhrN0W.png)
(https://i.imgur.com/BWZKYEm.png)
(https://i.imgur.com/a5vZuIn.png)
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I did testing today with two buffer x-rings in the balance chamber. Yesterday’s and today’s sessions were shot until my 45min SCBA tank dropped below 3000psi. No failures in the valve, though I have not yet taken it back apart for visual inspection after today’s session. It was still doing fine at the end of today’s session. Velocity was a little higher than yesterday (as expected). The velocity is highly pressure dependent, and the bottle reg is not doing me any favors as it creep after sitting and recovers slow when shooting. Next session I’ll tether the plenum directly to the SCBA tank and manually fill to desired pressure. I’m looking for a high quality in-line reg to user for tethering in the future.
I did some accuracy testing with three different slugs. I used up what I had cast of the Arsenal 151 and 169gr today, so I’ll be doing a long casting session before the next outing. The Arsenal slugs I shot today were unweighed/unsorted, all lubed and sized. The NOE slugs were weight sorted to <+/- 0.5gr, lubed and sized.
It’s looking like 1100fps is too fast (at least for the 151gr) as the groups opened up quite a bit. I have not shot the 168gr that fast, and might not as shot-cycle/air usage/recoil all become objectionable. I’m now thinking that my longer term velocity settings might be in the 1050 to 1070fps range.
I shot some NUAH targets, set at 100m. Easy enough to get “Marksmen” level in most cases. Though I’ll need to get 200m and 300.m groups for a better evaluation.
For reference, my 7mm regularly does 1.5moa.
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I got my new scale that can measure over 150gr. The average weight of the Arsenal 170gr ended up at 171.1gr. I’ll keep everything from 170.6 to 171.6gr. There were not many >171.6, slugs with casting wrinkles tended to be less than 170.6gr.
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I don't have any data to confirm, but I think round O-rings would compress more than X-rings.... They should also be more "progresssive" in the force required, I think, as they will start out with minimum contact with the seats and each other, and have room to "spread out" into the square space they occupy.... Also, using the softest possible ones would be desirable, as the idea is to increase the "stopping distance" of the crash as far as possible....
Bob
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I got a tethering regulator and tried it out today. It is a used (2018, like new) Praxair ProStar PRS409249 for under $200. It’s a rare model from what I can tell. New, it cost about $1000. Almost 6 lbs. stainless steel body. 6000psi max input, 4500psi max output. Very stable over a wide input range.
Testing today was with a peek poppit. The Derin poppit had occasional valve lock No valve lock so far with the peek poppit. Tested up to 3600psi.
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NICE Reg !!!!
Bob
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Looks to be a high quality reg. And at a steal. Nice.
Dave
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Scott, if you are looking for long lengths of CF tubing to sleeve barrels, try Rockwest Composites.... https://www.rockwestcomposites.com/tubing/round-tubing/round-carbon-fiber-tubing .... Here is a partial list....
(https://hosting.photobucket.com/images/oo221/rsterne/Screenshot_2024-07-11_184540.png) (https://hosting.photobucket.com/images/oo221/rsterne/Screenshot_2024-07-11_184540.png)
They make all types, from Filament Wound to Twill Weave, and from Standard Modulas CF to Ultra High Modulas (VERY stiff).... and lengths up to 10 ft!.... Note the 2nd item on that list is 0.50" ID x 1.155" OD (0.328" wall), it is up to 8 ft. long, and only $102.00 (per foot)....
Bob
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I spent the day with my friend at his 200 acre ranch in the Cambria hills. I brought my 7mm and new .308 builds. First thing on the agenda was to do a short range accuracy comparison between the two rifles. We each shot a five shot group at 100yds. John is probably a better shot than me and he was shooting the 7mm, which has already been proven to be a good long range airgun. He shot an Expert (almost Master) group on a N.U.A.H. target. I shot a Marksman (almost Sharpshooter) group with the .308. After that, we proceeded with the main event. We setup a 25”x23”x0.5” steel plate target at 600yds with a hit indicator light. It took a couple of hours to locate and access (via UTV) a suitable area on the property, and the setup the target. We used my 7mm and 308 builds to shoot. I guess the steel plate was too heavy as we rarely saw a hit indicated with the 105gr 7mm, but occasional hits indicated with the 171gr 308. After lunch, we drove the UTV back down the valley and saw that we had actually been hitting the target more than we thought. 6 hits with the 308 and 8 hits with the 7mm. The larger impacts mostly in the upper half were probably from the 308, and the smaller hits mostly in the lower half were probably from the 7mm. I plan on setting up a 12”x12”x3/8” steel plate next to the larger plate. That lighter weight plate should allow better activation of the hit indicator.
I’m still having issues with the bottle reg (lag and creep) on the .308. So I decided to go ahead with the dual bottle arrangement and also to integrate the Praxair regulator into the rifle. The additional bottle and heavy regulator will add another 9lbs. The gun is already way too heavy, so I don’t even care all that much about the extra weight.
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I was using a sub $100 Aliexpress bottle regulator. Although it was a decent fit for this build, it just does not have the consistency that I wanted. I even had it apart, replaced some o-rings, removed a couple of split Belleville washers, re-shimmed and re-lubed it. Still not great.
I came up with a new solution using dual bottles (which I wanted anyway) and the highest quality regulator. With big, quality gauges. I’m still waiting for delivery of some M18x1.5 all-thread so I can finish my dual bottle adapter. Four liters of air on board, at 4500 psi. More shots per fill. Better pressure regulation. A lot more weight.
I was doing some final polishing of the 61” TJ barrel and noticed a loose spot inside, about 12” from the muzzle. It measured 0.004” excess diameter and about 1.5” long. I have no idea how that could happen with only 4000psi air, and I don’t think it was there when I got the barrel. So I cut off 12” and now have a ~49” barrel. That means I’ll need about 50psi more to maintain the desired velocity. But I’m guessing that leaking air past the loose spot was causing a velocity loss anyway. The rifle is now about the same length as my 7mm. Here are some progress pictures.
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It’s hard to imagine the air pressure would cause the .004” spot. About the only way I can think would be if you had a squib at some point and ran another behind it. Was the outside of barrel bulged?
You definitely have plenty air on board now :D
Dave
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That has to the the biggest regulator anybody has ever mounted on an airgun!.... :o 8)
Bob
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It’s hard to imagine the air pressure would cause the .004” spot. About the only way I can think would be if you had a squib at some point and ran another behind it. Was the outside of barrel bulged?
You definitely have plenty air on board now :D
Dave
In the case of firearms, firing another round with a squib stuck in the barrel can be dangerous. It can easily bulge the barrel or even blow it open. Big pressure difference when gun powder burns in a closed container. But in an airgun?
I’m trying to envision how an airgun could develop the 40,000psi+ that it would take to bulge the barrel. How about this: squib slug stops before exiting. Another shot is fired. Momentum in that new slug compresses the air in front of it faster than the squib can accelerate out of the way? Sounds far fetched, but at least plausible. I’m trying to understand how it happened. I think it’s plausible. Any thoughts from anyone appreciated.
That part of the liner is stuck in the DOM tube part that I cut off. So it’s most likely bulged on the outside of the liner.
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That was the reason I thought was the momentum of the next slug compressing the air between.
Like I said, it’s hard to imagine how with the relatively low pressures we are dealing with, that was just a guess. I just can’t think of anything else right now.
Dave
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That was the reason I thought was the momentum of the next slug compressing the air between.
Like I said, it’s hard to imagine how with the relatively low pressures we are dealing with, that was just a guess. I just can’t think of anything else right now.
Dave
In that scenario, we are compressing a 48” column of 14.7psi air as the newly fired slug converges on the squib.
So how does it create a 1.5” long bulge at 48” down the barrel? 48” of 14.7psi air squeezed down to 1.5” is only about 500psi. Squeezing it down to 40,000psi, that’s only 0.018” long. No where near 1.5” long.
Now looking for another explanation for the bulge.
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Weak steel in a part of the barrel... I had a .457 barrel bulge from a stuck slug...
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That was the reason I thought was the momentum of the next slug compressing the air between.
Like I said, it’s hard to imagine how with the relatively low pressures we are dealing with, that was just a guess. I just can’t think of anything else right now.
Dave
In that scenario, we are compressing a 48” column of 14.7psi air as the newly fired slug converges on the squib.
So how does it create a 1.5” long bulge at 48” down the barrel? 48” of 14.7psi air squeezed down to 1.5” is only about 500psi. Squeezing it down to 40,000psi, that’s only 0.018” long. No where near 1.5” long.
Now looking for another explanation for the bulge.
I’m not sure of the full dynamics of how a barrel obstruction causes the bulge but I know it’s quite common. Even with .22 shorts. Keep us updated. It doesn’t sound like you’ll be loosing much performance with the barrel trim, might even help in the harmonic department.
ETA: maybe it’s the lead itself causing the expansion similar to the pressure inside a swage die being transferred to the sides. ?
Dave
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That was the reason I thought was the momentum of the next slug compressing the air between.
Like I said, it’s hard to imagine how with the relatively low pressures we are dealing with, that was just a guess. I just can’t think of anything else right now.
Dave
In that scenario, we are compressing a 48” column of 14.7psi air as the newly fired slug converges on the squib.
So how does it create a 1.5” long bulge at 48” down the barrel? 48” of 14.7psi air squeezed down to 1.5” is only about 500psi. Squeezing it down to 40,000psi, that’s only 0.018” long. No where near 1.5” long.
Now looking for another explanation for the bulge.
I’m not sure of the full dynamics of how a barrel obstruction causes the bulge but I know it’s quite common. Even with .22 shorts. Keep us updated. It doesn’t sound like you’ll be loosing much performance with the barrel trim, might even help in the harmonic department.
ETA: maybe it’s the lead itself causing the expansion similar to the pressure inside a swage die being transferred to the sides. ?
Dave
The lead itself would have to expand to 0.004” bigger to make the bulge. I would think that would really make it stuck in the barrel. If there was a squib, it got fired out with the next shot. Nothing was ever stuck.
So the two slugs collide in the barrel and make a 1.5” long bulge. And then get squeezed back down and continue out the muzzle. That’s another plausible theory.
From now on I’ll keep a long rod in the gun case. And push any squibs out the breech with the rod rather than out the muzzle with air. Squibs happen in an airgun when there is a partial valve lock. I just installed a more powerful solenoid, so hopefully less chance of a squib.
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I know Marco had a barrel bulge from a squib....
Bob
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I know Marck had a barrel bulge from a squib....
Bob
I believe it and have already accepted as fact that it can happen in an airgun. I learned something new.
I had planned to possibly shorten the barrel anyway., so cutting it was not a big problem. Even though the 61” barrel tests ended prematurely, I still have sufficient data for comparison between 61” and 49”. So more to learn.
Since I shortened the barrel, I now have room in the large gun case to add a moderator which I’m working on right now. I’m still waiting for more hardware for my dual bottle adapter.
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I finally got the dual bottle arrangement built. After breaking 2 bits on some stainless all-thread, I made the banjo bolts from two 10.9 (similar to grade 8 ) M18x1.5 bolts. I would have preferred 8.8 (general purpose), but M18x1.5 bolts have a limited selection. I also bought a set of cobalt drill bits to make it a little easier. I decided to use 1/8” bits rather than smaller to reduce the chance of breakage. For safety, I prefer smaller passages on the bottle side of the regulator. I repurposed two 3d printer nozzles as restrictor jets. I have foster quick connects on the regulator inlet and outlet. That makes assembly/disassembly of the airgun much simpler, but one of them has a leak, so I still need to deal with that. I think it looks better with the 49” barrel. There’s room in the case, so I might add a big moderator but then it will be back to looking too long.
I hope to get back to the range next Friday or Saturday.
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Dual tanks looks good.
I think the shorter barrel looks better as well. It’s still an extreme length without getting into the ‘too’ long realm.
Will be curious to see how it does ‘out there’.
Dave
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I was able to get to the range today and do some testing with the shorter barrel (was 61”, now 49”).
First the bad news. It lost about 60fps in muzzle velocity. Prior, muzzle velocities at 3500psi with the 151gr and 171gr were 1106fps and 1076fps. Now they are 1045fps and 1014fps. Even with changes to my spreadsheet including reducing the coefficient of friction in the barrel, that velocity loss is still more than expected. I’ll need to look into why later on.
Now the good news. The integrated Praxair regulator is very consistent. In 22 shots with the 171gr, I got an average velocity of 1014.0fps with a standard deviation of 1.98fps.
I shot four groups of 5 shots each at 100m. One was about 1.4moa, but the other three were:
0.99moa
1.04moa
1.06moa
So, accuracy is a little better now.
Why?
Lower velocity?
Shorter barrel?
More weight (heavy reg and twin bottles)?
Weight distribution?
Reduced velocity spread?
With the new solenoid, valve opening is still unreliable when over 3600psi. So I’ll use 3500psi for now. So I’m limited to 1014fps with the 171gr. Less than I originally wanted, but I’ll live with it for now.
The 3500psi set pressure doesn’t give near as many shots either.
I might have TJ make me another 61” barrel in the future. Probably go with a carbon sleeve next time.
Also considering getting a .257 in 1:10 TJ barrel rather than the .308 1:10, but then I’d have to get Arsenal or Noe to make me a mold for a 3.5 or 4 caliber long BBT slug.
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I had originally used Foster quick disconnects to install the regulator. Not the best for a semi-permanent, semi-rigid installation.
I had to educate myself on the various swivel fittings that might work. I ordered a few likely candidate to look at.
First I considered flange seals. They use an o-ring seal. But too bulky. Better for high flow rates, but that’s not needed for a plenum refresh.
I also looked at ORFS (o-ring face seal).
I looked at NPSM. This is an adaptation of NPT to a conical metal to metal seal.
I ended up using JIC connection. The primary USA conical metal to metal seal, with a little higher pressure rating than NPSM.
These connections are more typically used in hydraulics but can work for gases too. Time will tell.
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Absolutely on FM Flare to M taper compression fittings ! As i recall these are rated when in stainless far above the pressure we use / see in our PCP's.
I grew up playing with these as my father worked in Rocketry hydraulics always having boxes of misc type fittings he had taken off site having at home he used for far less critical applications.
I too cobbled stuff together playing as his rather ignorant mini-me. Lol
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Brennan JIC in these small sizes are rated at 6000psi for steel (what I got), and 7200psi for stainless.
There are higher rated, higher cost AN versions with tighter tolerances. Might have been what you played with. Can be used interchangeably with JIC but at the lower rating when combined.
JIC, industrial and automotive
AN, aerospace and military
I’m familiar with pipe threads, but these other fittings are mostly new to me. I was aware of them but not familiar.
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Now that I’m willing to accept a lower velocity with the 171gr .308, I tested more for best groups today. Cambria range at 2000ft elevation. I tested down to about 975fps. 1000 to 1010fps range was looking good. Best 1005fps group was about 0.64moa C-to-C, at 100yds. Vertical spread was only 0.25moa. I used a front bipod and rear squeeze bag.
I don’t remember ever doing better than that at 100yds.
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That’s looking really good, Scott. Thumbs up.
Dave
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WOW, just WOW, Scott !!!!
Velocity is cool, but accuracy is everything !!!! .... Looks like Arsenal Molds are pretty decent, HUH?.... 8)
BTW, TJ's have a 7" twist available in .257 cal, I had them make a mandrel for it years ago for the 3.5 & 4.0 cal. long BBT's.... and NOE have molds for both for the original designs.... I don't know if a 10" twist would handle the 4.0 cal, but probably would the 3.5 cal.... I know for a fact both tumble in a 14".... have not tried them in a 10" yet....
Bob
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WOW, just WOW, Scott !!!!
Velocity is cool, but accuracy is everything !!!! .... Looks like Arsenal Molds are pretty decent, HUH?.... 8)
BTW, TJ's have a 7" twist available in .257 cal, I had them make a mandrel for it years ago for the 3.5 & 4.0 cal. long BBT's.... and NOE have molds for both for the original designs.... I don't know if a 10" twist would handle the 4.0 cal, but probably would the 3.5 cal.... I know for a fact both tumble in a 14".... have not tried them in a 10" yet....
Bob
The 3.5 and 4.0 caliber .257 molds at NOE have been “SOLD OUT” for a long time (at least a year?)
I don’t see the 1:7 in the TJ price lists. But I heard rumors of a 1:7 and 1:8 in .257 caliber.
The 4.0 caliber long .257 has a predicted BC that is not much better than the 3.5 caliber long .308. The .257 would save on air and lead, however, it might be a safer bet to stick with the .308, 3.5 long (171gr).
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That's the way to go, good shooting. My best results at 100M have also been around 1000-1010 fps, 1/2" ctc was the best group.
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Wow! That is absolutely FANTASTIC!!!
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I had a brief session at the range today to try the .308 at 200m. I set 4 paper targets at the top of the 200m berm. Baseline was to be the Arsenal 171gr that did well at 100yds. The 222yd group was around 2moa. So I switched to the Arsenal 152gr and shot three groups. Best group shown below was 0.62moa c-to-c. That’s the best 200yd+ group that I have ever done. Wind was low. I really wanted the the higher BC 171gr to be my primary .308 projectile. But the 152gr might have the accuracy edge. And its BC is plenty good for 300yds. I also need to have another casting session and bring the NOE 152gr back fior comparison.
The outside edge of the black circle is 1moa at that distance.
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In terms of equivalent stability, for the same shape slug, the twist rate is proportional to the calibre.... Therefore, a .257 that is 3.5 cal. long would require a twist rate (0.257/0.308) = 0.83 times the twist required in a .308 for the same length & shape slug.... In other words, a 12" twist in .308 works like a 10" twist in .257....
I don't know about an 8", but for sure TJs have a 7" mandrel in .257 cal....
Bob
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In terms of equivalent stability, for the same shape slug, the twist rate is proportional to the calibre.... Therefore, a .257 that is 3.5 cal. long would require a twist rate (0.257/0.308) = 0.83 times the twist required in a .308 for the same length & shape slug.... In other words, a 12" twist in .308 works like a 10" twist in .257....
I don't know about an 8", but for sure TJs have a 7" mandrel in .257 cal....
Bob
And wouldn’t a .257 at 3.5 calibers long (0.900”), require the same twist a .308 at 3.5 calibers long (1.078”)?
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If you are talking twist rate IN CALIBRES, then yes.... but in inches the twist rate is proportional to the calibre.... so 0.83 times for a .257 compared to a .308.... ie a 10" twist in .257 is the equivalent of a 12" in .308....
Bob
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Not sure I follow.
A 2.5 caliber long .257 works well in a 1:14 twist.
Scale that up to a .308 in all dimensions and it should also work well in a 1:14 twist. True?
1:14 for any other caliber that uses that scaled projectiles that is 2.5 calibers long. Correct?
The .257 is 0.643” long. The .308 is 0.770” long.
If the .257 weighs about 78gr, the .308 will weigh about 134gr.
Proportionally, the same slug. So the same twist rate. Is that right?
Or, would the .308 use a slower twist rate in this slug instance?
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For a given proportion of projectile (eg. 2.5 calibres long), the minimum twist rate for stability IN CALIBRES would be the same.... However, just as the projectile in a larger caliber is longer (in inches), so is the minimum twist rate greater (in inches).... Larger calibres naturally have a longer twist rate, and vice versa....
Using the Kolbe Twist Calculator, at 1000 fps I get the following for a stability of 1.500....
.257 cal. 101 gr. 0.900" long.... 8.4"
.308 cal. 176 gr. 1.078" long.... 10.0"
Note both slugs are 3.5 calibres long and the same shape.... http://www.geoffrey-kolbe.com/barrel_twist.htm
Bob
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For a given proportion of projectile (eg. 2.5 calibres long), the minimum twist rate for stability IN CALIBRES would be the same.... However, just as the projectile in a larger caliber is longer (in inches), so is the minimum twist rate greater (in inches).... Larger calibres naturally have a longer twist rate, and vice versa....
Using the Kolbe Twist Calculator, at 1000 fps I get the following for a stability of 1.500....
.257 cal. 101 gr. 0.900" long.... 8.4"
.308 cal. 176 gr. 1.078" long.... 10.0"
Note both slugs are 3.5 calibres long and the same shape.... http://www.geoffrey-kolbe.com/barrel_twist.htm
Bob
Thanks Bob, I thought they would have the same twist rate. My assumptions were wrong. I learned something new today. I do often use the Kolbe twist form and bullet design form calculators.
http://www.geoffrey-kolbe.com/barrel_twist.htm
http://www.geoffrey-kolbe.com/drag.htm
In the second one, the derived data for twist didn’t seem to match well with the twist calculator, at sub-sonic to mach1. So I had come up with my own assumptions, without any real verification (except some empirical data).
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Yes, the 2nd calculator is just an approximation.... The Twist calculator shows what happens around Mach 1 much better.... The worst stability is just below Mach 1, requiring the fastest twist.... If a bullet is stable there, it will work anywhere....
Bob
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Yes, the 2nd calculator is just an approximation.... The Twist calculator shows what happens around Mach 1 much better.... The worst stability is just below Mach 1, requiring the fastest twist.... If a bullet is stable there, it will work anywhere....
Bob
And that’s where I like to see a 1.5 stability factor. Right at that inflection point.
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Can't argue against that!.... Pretty much guaranteed plenty of stability and therefore the best BC at whatever velocity you choose.... One thing to consider, though, is that the Kolbe Twist Calculator is probably somewhat conservative at the "below Mach" inflection point when working with boattails.... Miles Morris tells me you can probably get away with a slower twist on a BBT than the Kolbe minimum, and actual testing by quite a few shooters confirms that.... Imagine if you will rounding off the lower spike on the curve starting at 1000 fps, dipping about half as far as the spike, and then rejoining the vertical line at about the height of the 1000 fps point.... Something like this....
(https://hosting.photobucket.com/images/oo221/rsterne/Screenshot_2024-10-03_112514.jpg) (https://hosting.photobucket.com/images/oo221/rsterne/Screenshot_2024-10-03_112514.jpg)
In other words, about a 10% slower twist than the inflection point.... Unfortunately, each slug will be a bit different, but it's something to keep in mind if you are wondering if a given slug will be stable in a given twist....
Bob
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Bob,
Also, in a marginal twist rate situation, a slower muzzle velocity could help stability. The graph for that example shows 1:7.5 twist for 1060fps but only a 1:9 twist for 900fps.
My preference for ELR is 1000fps+, but I know some others having great success out to 300.yds at under 900fps.
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I got my 73” TJ barrel liner prepped and tested.
Performance definitely better. With the regulator gauge reading 3000psi (it reads high, so actual pressure closer to 2900psi). 171gr at 1060fps and 151gr at 1090fps.
I used a 1” OD aluminum sleeve over the barrel and that saved about 8lb compared to the steel sleeve. Accuracy was about 2moa at 100m. So I’ll be finishing the 1” OD steel sleeve to compare. Might just be the barrel prep, so will also work on that some more.
Another part of this project for 2025 will be an attempt at making my own barrels. I ordered and received a button to form 1:8 twist .308 rifling. I’m going to convert my hydraulic pipe bender into a press for pushing the button. There will likely be some trial and error experimentation.
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Accuracy about the same with aluminum or steel barrel sleeve.
I got some efficiency numbers. The dual 2L bottles give about 60 shots when going from 4000 to 3000psi. And then a few more shots just off the regulator. That long barrel gives good efficiency. I’m starting to think that the slow acceleration of a heavy projectile in a very long barrel is not conducive to good accuracy. The projectile spends a long time in the barrel after the trigger is pulled.
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I’m posting an update on my progress. Well, for the last year, I’ve been having back issues (and more). I decided back in December that dealing with a “super heavy” bench gun was not good for the long term. I spent the last 6 weeks re-making the square tube in aluminum and the barrel housing out of carbon fiber. I also removed the 2lb weight in the stock. I’ve reduced the weight about 15lbs in total. That’s where it stands as of today. I could cut a couple more lbs by going to 2024 and 7075 aluminum for the plenum, valve, and end-cap. Also, could go back to one bottle. That’s for later consideration.
I plan on shooting it again (finally) on Monday.
I’ve got some more projects in the works for this year. A few of them related to this build. When I rebuilt it, I increased the maximum projectile length to 1.25”. I should now be able to fit 200gr or even heavier slugs, and 500fpe should be doable. Except, even though the plumbing and plenum is designed for 3900psi, I can’t reliably open the valve at pressures much over 3200psi. I’m otherwise happy with the electronic hammer at 3000psi to 3200psi. The balance piston is already too big in diameter, as it won’t reliably seal at 3000psi with the PEEK poppit. It leaks more than I want to deal with. So a project I’m planning is a piloted valve for this rifle. Hopefully opening the valve more easily at higher pressures, while still allowing a good seal at the valve face.
I want to try a faster twist than the current 1:10 TJ barrel, so another project I’m working on is to make my own barrels. First will be a .308 to compare with the TJ? And then a 6.5mm.
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Looking forward to viewing this redesign ... perhaps Oregon in May ?
Be well my friend ;D
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Will be interesting to see what pilot design approach you use. I enjoy the different ones I use.
Dave
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Looking forward to viewing this redesign ... perhaps Oregon in May ?
Be well my friend ;D
I do plan on using it at the Oregon match in May. Though my fallback will be to use my 7mm.
Cameron K and I are planning on shooting at the Sacramento club’s February 8th long range match. If the testing goes well on Monday, I’ll have the .308 at that match. I’ll bring my 7mm as well.
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Looking forward to viewing this redesign ... perhaps Oregon in May ?
Be well my friend ;D
I do plan on using it at the Oregon match in May. Though my fallback will be to use my 7mm.
Cameron K and I are planning on shooting at the Sacramento club’s February 8th long range match. If the testing goes well on Monday, I’ll have the .308 at that match. I’ll bring my 7mm as well.
I just may need to join you hooligans and go lob some lead.
Scott, if needing a place to stay over ... doors always open ;)
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Scott S, Cameron K and I had a good time at the Sacramento club long range match a couple weeks ago. Since then, I bought a Lee mold for a 300 Blackout. The as cast slugs are 1.36” long. I can fit up to 1.25” long in the latest build. I made a jig and milled .06” off each end of a bunch of the slugs for testing. The final length was 1.243”, and finished weight was 218gr. At 3500psi, I was getting 1018fps average for a little over 500fpe. That’s a milestone. I had some partial valve luck trying to go higher and got one stuck in the barrel. Took quite a bit to knock it out with the custom extra long cleaning rod. Accuracy at 100yds was not great (about 3moa), and neither was the BC. I suspect that those slugs are too much for the 1:10 twist barrel. I did a bunch more testing of the 151gr and 171gr BBT at 1020fps to 1060fps. Still getting no better than about 2moa average with this 73” barrel.
Took it apart when I got home and plan on starting on the piloted valve tomorrow.
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Shorter barrel gives better accuracy? I've had issues with long barrel and inadequate support, going for double CF tube around the liner and barrel band that very tightly attaches barrel to the air tube solved that. Any ways to stiffen your barrel more? Alternatively adding weight to the muzzle end could calm down the vibrations.