Tale of THREE BRods - .25, .30 and .357 Completed, Page 19 !

[grand-galop]

rstern!! Can you tell me the advantages of bottling the gun or doing something like double tubing would be more stiff construction...
I know you have more than one rifle with bottle and i know the power mods you make to them called for more volume..  But what the advantages of bottling over double tubing or increasing tube size like the FT model??

[rsterne]

The bottle I am using is 500 cc, plus the 80 cc in the plenum gives a total air on board of 580 cc.... A stock MRod is 215 cc, so this is 2.7 times the air.... Even if you used a second tube of the same diameter as the original, you could only double the volume. leaving the bottle with 1/3 more air.... The reservoir portion of the MRod tube (just the part between the fill fitting and the gauge block), which is 15" long, weighs 1.5 lbs., and a second reservoir of the same volume, with the extra ~3" of length needed for end plugs, and the end plugs themselves, would weigh about 2 lbs.... for a total of 3.75lbs. for 430 cc.... The bottle I have (aluminum, not CF) weighs 1.75 lbs., to which you have to add the tank block and 4.75" of tubing for the reservoir (the weight of the rest of the tubing is the same as in an MRod), which is about 1 lb., for a total of 2.75 lbs.... so the bottle setup is 1 lb. lighter for 33% more air....

If you regulate, the gains in usable air volume are even greater.... You have to add the weight of the regulator to both guns (similar), but to get an 80 cc plenum like I have, and install the regulator (~ 20 cc) inside the upper tube, which you would do in a regulated MRod, you would lose about 100 cc, bringing the HPA reservoir down to just 115 cc, or 330 cc on a double tube setup.... Compare that to the 500 cc bottle (upstream of the regulator), and you have 1 lb. less weight and over 50% more air for the bottle gun, compared to a regulated double tube MRod with two identical tubes.... I don't think the stiffness of the reservoirs is an issue, because most would free float the barrel anyway....

Using a single, larger diameter tube instead of two smaller tubes, does NOT save any weight, if you stay with the same material.... The tube itself, in order to have the same MSWP and safety margin, must increase in thickness in proportion to its diameter, so the weight of metal to encompass a given volume actually stays the same.... If you continue that larger diameter back to encompass the action, then you have more weight for the single tube design.... Yes, you can use titanium tubing instead of steel for the tubes, but you can also use a Carbon Fibre bottle instead of aluminum.... Both will save a lot of weight, but of course are much more expensive....

Bob

[rsterne]

I started working on the receivers today.... On most of my projects I have to make them from scratch, but for this project I started with two .25 cal MRod receivers and bolts.... The first thing to be done was to drill out the transfer port hole to 3/8" OD, the same diameter as the Cothran valve, and the holes I drilled in the tubes to match.... I plan to use a piece of 3/8" OD Teflon rod, drilled as appropriate, for the transfer port.... So, I set one receiver up on my milling attachment on my lathe, using a stop at one end to minimize the setup on the other one, centered off the existing transfer port hole, and drilled it out to 3/8".... I then took it out of the vice on the milling attachment, put the other one in, checked the hole center, and drilled it as well....

I had decided to drill the front of both receivers to 5/8" for about 1-3/8" deep, staying in front of the two 4-40 screw holes, so that the full diameter part of the barrel will be held rigidly in the receiver.... It doesn't do a lot of good to have a 5/8" barrel and then turn it down to 1/2", all it will do is flap around (bend) where the receiver starts.... I planned on using my 4-jaw chuck, holding the back of the receiver, and drilling the front with a 5/8" drill in the tailstock, but I found that the receiver is VERY flexible where it is machined away for the magazine, so much so I was afraid that it would not run true, and if the drill grabbed, it might bend and destroy the receiver, so I needed another plan.... I chucked a piece of 1/2" cold-rolled steel in the chuck, which was a perfect slide fit in the front hole of the stock receiver.... I then mounted the receiver in my milling attachment vice so that the steel rod rotated and slid back and forth freely, so that insured that the chuck was lined up with the barrel hole in the receiver.... I then simply drilled the hole out with a 5/8" drill to a depth of 1.4" to the shoulder of the drill.... The LW barrel is actually 16mm, which is a few thou over 5/8", and it slid perfectly into the drilled hole.... I drilled the other receiver the same way, and it turns out that the CF tubing I have is also 16mm OD, so it also fits nicely into the new, larger hole in the front of the receivers.... Here is what they look like now....



I then checked to make sure that the stamped end of the LW barrel was, indeed, the breech, by measuring the land diameter with my new small hole gauges.... I found out that the land diameter at the breech is 0.247" and at the muzzle it is 0.244", so the barrel has a 0.003" choke.... I machined the breech down to 1/2" OD for 1.5" of length, and checked how it fit into the receiver.... With an O-ring against the shoulder of the barrel, sitting against the drilled shoulder in the receiver, the back of the barrel was about 0.010" too far back for the magazine to slide into place, which I left for now.... I then used a chambering reamer I made several years ago for the LW barrel on my Hayabusa, and machined the chamber and leade to the required depth so that the front of the chamber is just ahead of where the front of the barrel port would be located.... I checked what a bullet and pellet looked like chambered to that depth, and was pleased with the results, so carried on with the next step, which was laying out the barrel port....

I carefully measured the location of the port from where the back of the barrel needed to be to fit against the locating hollow in the magazine, added a 0.010" (because I knew I had some extra length available), set the barrel up in my milling attachment, carefully centered it relative to the chuck, and drilled and milled the barrel port and the 3/8" diameter flat for the transfer port to seal against.... The port transitions from a 1/4" diameter at the TP spot face to 3/16" wide by 5/16" long at the bore line, so I actually can use up to a bore size transfer port should it be required.... After the milling, I touched up the port with a small ball in my Dremel, blending the 1/4" round port into the oblong barrel port, within the thickness of the barrel itself.... I then slid the barrel into the receiver, lined up the transfer port spot face with the hole in the receiver, and tried the magazine.... As expected, the barrel needed to be shortened 0.010" for the magazine to slide into place....

The last step on machining the back of the barrel was to machine the internal O-ring groove to seal the bolt.... If anyone else attempts this build, the location of that groove is CRITICAL.... The groove is 0.070" wide to fit the O-ring, and from the back of the TP spot face to the back of the barrel is only 0.125", so there is only 0.055" of material left TOTAL for BOTH sides of the groove.... The force on the O-ring from air pressure, is backwards, so I left more material there, 0.035", which only leaves 0.020" between the front of the O-ring groove and the back of the TP spot face.... and YES they would intersect if you put the groove too far forward in the barrel.... Anyway, knowing that was an issue, I was careful and all went well, the bolt slides nicely through the O-ring, and the barrel now looks like this....



I assembled all the parts so far, just so that I could have a look, and take a photo so that you can see the progress, and the proportions....



I'm very pleased with the progress to date, so I guess it's time to make the hammers and rear plug and SSG next....

Bob

[grand-galop]

Dang that looks good!!

[TerryH]

That looks really nice, Bob!

[Motorhead]

Very familiar work going on there .... measure 3 times & cut once.
Bob, your not wasting any time getting into this project !!! ... Looking great so far   

Curious to see what you do for Hammer & SSG type / design

[oldpro]

 Im trying to talk him into a MDS hammer on The Guild, I think he needs your source as McMaster wont ship to canada

[Motorhead]

Im trying to talk him into a MDS hammer on The Guild, I think he needs your source as McMaster wont ship to canada

Tell me it's not so ... Lol 

[rsterne]

The plan today was to get started on the hammers, but I decided instead to finish off the barrel and receiver.... I marked the locations for the spot faces for the setscrews on the top of the barrel and drilled those.... I installed the bolt and discovered I was missing parts yet again (that's what happens your first time playing with parts from a gun you have never worked on before).... I had to make the small plate that prevents the bolt from over-rotating yet allows the handle to be left or right.... no big deal, just time.... Fortunately I still had some small 2-56 screws left over from my R/C Yacht days to install it.... Once I installed the cocking pin in the bolt I could check the position of the shoulder on the bolt and the length of the probe, and the shoulder was too far forward and the probe too short.... I had heard that the bolt was hardened, so I heated the front cherry red and let it cool slowly, but it was still too hard to drill for a new probe.... I figured it might be just case-hardened, so I ground the tip off the bolt, and sure enough that allowed me to get a bite with a small drill....

Mounting the bolt straight in the lathe chuck was tricky because of the handle and the short mounting surface around that, which is the only place to grab it.... I came up with a neat trick to hold it straight while I tightened the chuck up.... I held the bolt probe in the tailstock Jacob's chuck to hold the end of the bolt centered, ran the handle end into the 3-jaw until the handle touched the chuck face, between two of the jaws, and then tightened the chuck good and tight.... When I undid the tailstock chuck and backed it away and mounted a tiny center drill, it was perfectly centered on the end of the probe where I had ground away the case hardened surface.... I drilled a small center hole, and the changed to a 3/32" drill and drilled in about 3/4".... I then removed the bolt from the chuck and ground the probe off, back to the tapered shoulder, and ground the tapered portion off to get through the case hardening there as well.... Of course since I had previously drilled a hole for the new probe, I had a center hole so that I could use a 60* center in the tailstock and line up the bolt again in the 3-jaw in the headstock.... I checked that it was running straight (it was), and then faced off the shouldered portion of the bolt square.... I checked it for length, and decided I would just put a very slight conical taper on the face from the OD to the probe hole.... This puts the edge of the shoulder flush with the back of the barrel port.... I then used a piece of 3/32" drill shank, glued it into the hole in the bolt, and then cut it to length for the new probe.... I made the probe long enough to seat JSB Kings with the skirt flush with the front of the barrel port.... That puts the King Heavies about 1/16" further forward, because they have a shallower hollow in the base, and a cast bullet would sit with the base 1/8" ahead of the front of the barrel port.... You can see what the probe looks like in the photo below....



I calculated the remaining area for airflow around the probe, and it works out to the same as a 15/64" transfer port.... I will likely be using a 7/32" port, which is the same size as the exhaust port in my modded Mrod valve (0.219"), so unless I have to go to full area porting should offer no restriction to the flow.... If I do have to use full 1/4" ports, I may have to change to a flat nosed bolt that retracts into a J-slot to allow maximum flow.... but I don't anticipate it, as I'm not trying to build a 100 FPE quarterbore....

At the muzzle I wanted to mount a Hatsan Air Stripper, as I happened to have a spare .25 cal on hand.... This required turning the barrel and threading it 1/2"-20 NF for the stripper.... I found a position in my 3-jaw where the bore ran true, and then faced the muzzle off square to the bore, and then turned the barrel OD down to 0.50" for a length of 0.80" and threaded it.... The photo below shows the installation....



Anyway, I think the upper assembly for the .25 cal version is now complete, and I can get on with the hammer and spring arrangement....

Bob

[rsterne]

I got a chance this afternoon to make the hammers.... I turned them from 1144 Steel.... They are 1.20" long, drilled 0.50" to a depth of 1.00" to the drill shoulder, and weigh 104 grams each.... There is just enough room in the cocking slot to go from contacting the back of the modded valve I made (which was shortened 0.075") to just cocking the trigger, for a total hammer stroke of 0.88" with my valve.... and about 3/4" with the Cothran valve.... With the deeper recess for the spring I have 2.75" of length from the shoulder in the hole to the back of the tube.... This means I can use a 3" spring for my SSG without having to make the rear plug stick out more than 1" behind the tube or so.... I'm pleased with the way they came out....



You may wonder why I am always using heavy hammers with maximum stroke in my builds.... Most of the time I'm going for a lot of FPE with a heavy bullet, and that means I need a large valve throat and lots of dwell.... The hammer energy to crack the valve and create lift comes from the spring and travel, but the hammer momentum to create more dwell comes from hammer weight.... A heavier hammer allows me to use a lighter spring, with a lower cocking force, and still get the FPE I want....

Bob

[grand-galop]

TONS OF info will be revealed in this build too...   LOL  Good to know the Relationship betwen the force needed to crack open the valve and the weight of the striker.... Thanks Bob

[aceflier]

Looking good Bob! Those are going to be a pair of awesome rifles. Can't wait to see how they shoot.

[oldpro]

 Its winter for sure its the only time Bob gets his builds going. Keep up the good work and looking forward to finished products.

[rsterne]

Today I made the first SSG.... I left all the parts longer than necessary until I find out if it will work properly throughout the range of tuning I need, I can always shorten them up and make them prettier later.... The reason I didn't make the second one at the same time is that the housing will incorporate a mount for my AR style stock....



The housing is a piece of 1.25" OD 6061-T6 aluminum turned down to fit in the back of the main tube, with the appropriate tapped holes to secure it.... On the top are two 4-40 screws through the receiver on 0.688" centers, and on the bottom a single 8-32 hole that mounts the trigger group.... The front just clears the back of the hammer when it is all the way back.... It is drilled through and tapped 9/16"-18 NF for the gap adjusting bolt.... I know that is huge, but it allows the spring assembly to be withdrawn out the back without any other disassembly.... That is a feature I always like to employ on my RVAs and SSGs, it makes changes a breeze.... The gap adjusting bolt has a spring seat turned on the front end, to hold the spring in alignment.... The stop rod part of the spring guide is a 4.5" long piece of 3/16" drill rod, threaded 10-32 both ends.... At the back it has quite a bit of thread for preload adjustment, which is provided by the two nuts locked against each other.... There is a # 008 O-ring on the shaft to act as a cushion when the guide comes to a halt.... The front part of the guide is turned from a piece of aluminum rod....It is 0.48" OD at the front (the same as the spring) and profiled to fit the bottom of the hole in the hammer.... Behind that is a 1" long section that is 0.355" OD to act as a spring guide.... There is enough space between the two parts of the guide to allow for more preload and cocking distance than I need.... The spring I am using at the moment is 3" long (instead of the 2.5" used in a stock MRod), with a rate of 13 lb/in.... I have 0.40" of preload at the moment, which is about 5 lbs.... If I need more, I just wind the nuts down on the shaft.... If I need less, I can shorten the gap adjusting bolt (moving the rear spring seat back), but a better solution would be a weaker hammer spring.... I just don't have anything lighter at the moment....



In the photo above, the SSG is adjusted for zero gap with the current 0.40" of preload with my valve.... With the Cothran valve, where the end of the stem sits further back, the gap adjusting bolt would be further back.... I can shorten the back of the SSG housing to allow more adjustment if I have to increase the preload much.... I'm thinking I'm getting pretty close to being able to tether the version with my valve in it.... That will answer a lot of questions about the hammer spring requirement at various pressures.... The Cothran valve will require less hammer strike than mine, at any given pressure, so I could use a lighter spring, or just increase the gap.... My valve at 3000 psi will be the test of the maximum hammer strike required for these guns....

Bob

[grand-galop]

Excuse my short memory Bob...  I see this setup and i dont remember if it was asked in the ssg tread...    Would a 1/4 to 3/8 piece of rc polytube could replace the 2 o-ring in the back... The détails and finish of making each of your milling is breath taking...I really like the race of that drilled bolt you got on the back..  It provides extra adjusting and possible extra strenght spring...  Great job!!

[oldpro]

Id flute those hammers as to not cause any air pocket in front of said hammers or vent air tube at rear of valve to allow air to get out. I find fluting works better and has added affect of less surface drag on hammer to tube.

[grand-galop]

Way to go Travis..

[rsterne]

Alain, there is only a single O-ring as a bumper at the back of the SSG rod, between the nuts and the adjusting bolt.... It just keeps the SSG from crashing metal-to-metal....

Travis, any air pressure between the hammer face and the valve simply vents out the sear slot and through the trigger.... It is possible, I suppose, that there could be a slight vacuum behind the hammer that slows it, but the hole for my SSG guide rod is a bit oversize.... I guess the only way to tell for sure would be to drill a vent hole in the rear plug or tube to see if it made any difference in the hammer strike.... I'm not saying that fluting the hammer is a bad thing, however, and I may take your advice on that....

I made the Teflon transfer port today and assembled the gun and fired it for the first time.... Tethered at 1900 psi, at zero gap on the SSG, the velocity was a bit low, so I increased the preload in two 0.10" stages from 0.40" to 0.60", at which point I was able to hit the velocity plateau at zero gap.... I then installed the action in the wooden stock.... It weighs right on 7 lbs. without the bottle....  I haven't done anything with the comb of the stock yet....



I then tested the velocity with 25.4 gr. JSB Kings and 34.2 gr. JSB Heavies, adjusting the Gap from zero out until the velocity dropped below 600 fps.... and then went the other way, adding preload (negative gap) until the plateau was confirmed.... ie I knew what the maximum possible velocity was with this valve, and 0.219" ports, at 1900 psi.... Here are those results....



As expected, adding any preload against the valve stem instantly increased the report.... I like to adjust the preload on my SSGs until I can reach the velocity plateau just as the gap goes to zero.... Any more preload and you are just making the gun harder to cock, and it means that you have to increase the gap to drop the velocity back down to the knee of the curve.... I want just a bit more power, I would like to be on the knee of the curve at about 950 fps with the 34 gr. Heavies, so with this valve I would need to increase the regulator setpoint slightly, I think 2000 psi should be about perfect.... However, before I make that decision, I want to test the other tube with the Cothran valve at the same pressure, to compare the results....

All in all, I'm delighted with the initial results.... With the SSG adjusted so that there is a tiny gap, I am at 944 fps (68 FPE) with the 34.2 gr. JSB Heavies, at only 1900 psi.... Moving the seat forward in the MRod valve so that I was able to achieve the 7/32" exhaust port was worth the effort....

Bob

[Machinist]

Nice work on everything.  I know you use what's on hand, but I think it would look good with a telescoping type stock.  Surprised the SSG rod is so long. I'm gonna make one soon, and figured it should be pretty long. 

Again, just nice work on everything.

Steve

I copied a lot of the info, explanations and charts for later. Thanks Bob.

[rsterne]

The SSG can be reduced in length at least 1/2".... by shortening the gap adjusting bolt, the housing, and the sliding rod.... Beyond that, you have to go carefully, or you may run out of room for preload and gap adjustment.... I left mine overlength on purpose, because the Cothran valve will require the entire SSG to be further aft about 1/8", as I had shortened my modded valve 0.075 and the poppet is in a different place, due to the valve seat being moved forward.... I probably should have linked to those mods earlier in this thread, I just assumed most would remember them....  http://www.gatewaytoairguns.org/GTA/index.php?topic=111416.

Bob