Regulated Disco Double

[Buldawg76]

Bob
I can attest to the frustration encountered in building and tuning race cars and bikes where changing one thing leads to 2 or 3 or more things that now have to be dealt with due to the one change that was supposed to be quick and simple. It never goes exactly as planned for the most part but its that ability to work thru and recognize the needed extra work before it becomes a disaster or damaged beyond the point of salvage or repair that gives us the best satisfaction of accomplishing what we started to do in the first place.

More work very well done on your part. It looks to be getting closer and closer everyday now.

Mike

[rsterne]

Well, today was the day!.... I spent the morning doing the final barrel prep work.... I crowned the muzzle, lapped the bore with JS Bore Paste, and then followed that by a polish with JB Bore Brite.... It now gleams from end to end.... I then installed it into the receiver and installed the bolt.... I slid on the front breech retaining band, and installed the PRod trigger group, then slid the hammer into place and installed my borrowed SSG assembly, which I set to minimum gap.... The last step was to measure the length required for the transfer port, and make one from a piece of 5/16" Teflon rod.... I drilled it out to 7/32", and carefully faced the end off until it was 0.010" longer than the space, so it would have a bit of "crush" to seal properly.... Bolting down the receiver was the last task before firing the first shot.... Well, except to take a photo....



As you can see, I haven't fitted the shroud, or modded the stock yet, those items will come after initial testing and tuning.... The gauge said the regulator setpoint was 1900 psi, so I loaded up a magazine, and tried the 34 gr. JSB Beasts....They fed fine, and the first shot was 867 fps (57 FPE).... Not bad for 1900 psi.... I shot a few of each weight pellet, and then shot the 25.3 gr. Monsters until the velocity started to change, and it actually went up about 20 fps at just under 1800 psi, so the setpoint is a bit lower than the gauge reads (or vice versa).... With the pressure a couple of hundred psi below the setpoint, I was able to increase the regulator adjustment, I turned the screw about 45* CCW, and refilled the lower reservoir.... The gauge now read nearly 2200 psi, so I repeated the procedure with the higher setpoint.... Here are the initial results.... The 18.1 and 25.3 gr. are JSB Heavies and Monsters.... the 27.4 and 29.6 gr. are my new BBT HP and FN cast bullets.... and the 34.1 gr. are the new JSB Beasts....



As you can see, the 18.1 gr. JSB Heavies were Supersonic, accompanied by the typical loud CRACK!!!.... The were the only pellet that shot faster at 2100 psi than at 1800, which tells me that I don't have enough hammer strike at the higher pressure.... Since I am out of adjustment with the QB spring (it is right at coil bind with no gap), I will need to come up with a different spring combination, or forego the SSG and just use the QB spring in a conventional preloaded arrangement for now.... I think I will do the latter, at least until I can establish what the maximum velocities are with the various pellets at the current setpoint.... The BBTs, both HP and FN, exceeded 60 FPE at 1800 psi, and the 25.3 gr Monsters were very close to that.... and it would seem that there is more there if I increase the hammer strike.... That certainly means that the gun is breathing well, with the bore-size porting.... Those are pretty healthy FPE numbers at such a low pressure, so I'm encouraged.... and I'm looking forward to finding out what the limit is at the ~2100 psi setpoint once I add some more hammer strike to find out where the plateau is with the different pellet weights....

Bob

[Machinist]

It's just amazing how long everything takes.  Figuring out dimensions, setting stuff up, making or modifying tools.  People think bing, bang, boom it all gets done in no time.  Every part is a one off.  Great engineering, awesome calculations, planning and execution Bob!

Can't wait to see some shot strings.

Steve

[rsterne]

The current plan is to test with 18.1 gr. JSB Heavies at various pressures to allow me to put Lloyd's Regulator through its paces before I change the SSG setup for the heavier bullets... There is plenty of hammer strike for over 2000 psi with those pellets, and it will be interesting to see what the plateau (maximum) velocity is at various pressures down to 1000 psi or so.... Hopefully tomorrow, I got sidetracked by the Missus today.... *LOL*....

Bob

[rsterne]

I tried four different pressures, each 1/4 turn apart on the regulator adjusting screw, and at each pressure I started at zero gap on the SSG and then increased the gap a turn at a time until the velocity dropped below 600 fps at each pressure.... I then plotted the velocity vs. gap at each pressure, with the following results....



The regulator setpoint dropped about 450 psi for 1/4 turn from 2150 down to 1700, then another 450 down to 1250, and then 350 psi from there for the last 1/4 turn, down to 900 psi.... At the highest pressure, even at zero gap, there is not enough hammer strike to reach the velocity plateau.... At 1700 psi, it is just about on the plateau at zero gap.... In both cases the maximum velocity was supersonic with the 18.1 gr. JSB Heavies.... Breaking Mach 1 on only 1700 psi is pretty impressive.... 

At 1700 psi, the plateau appears to be at about 1150 fps,  the knee of the curve was at about 1-2 turns of gap, and the downslope started at about 3 turns out at ~1000 fps.... At 1250 psi, the plateau was 1051 fps, and the knee of the curve occurred at 3-4 turns of gap, with the downslope starting at about 5 turns out at ~900 fps.... With the pressure at 900 psi, the plateau was 956 fps (37 FPE), the knee occurred at about 5 turns of gap, and the downslope began at about 6 turns of gap, at ~800 fps....

The regulator is very easy to adjust, and 100 psi increments should be relatively easy to obtain.... From 1250 to 2150 psi was 1/2 turn of adjustment, so each "hour" on a clock face would be about 150 psi.... That works out to about 50 psi for each 10 degrees.... The regulator seemed to have good stability, returning to the same setpoint after each shot, as close as I can read the gauge, which is a 3000 psi, 180* sweep, 5% gauge, with 200 psi increments.... I am delighted with the performance of the regulator.... and equally pleased with the ability of this gun to reach 950 fps with 18.1 gr. JSB Heavies at only 900 psi....

Bob

[rsterne]

I reset the regulator to 1150 psi, filled to 2000 psi, and shot 10 shots with the 18.1 gr. JSB Heavies with 3 turn of gap on the SSG and recorded the pressure drop.... I then repeated that at 4, 5, and 6 turns of gap, with the following results....



At 3 turns, the average velocity was 1018 fps (42.4 FPE) at an efficiency of 1.28 FPE/CI.... At 4 turns of gap, the average velocity was 1003 fps (40.5 FPE) at 1.43 FPE/CI.... With the gap increased to 5 turns, the velocity averaged 960 fps (37.1 FPE) at 1.64 FPE/CI.... and at 6 turns the average was 844 fps (28.6 FPE) at 1.95 FPE/CI.... Remember, this is at only 1150 psi.... The ES over the 10 shots was around 14-15 fps for most strings, although for the one at 4T out it was only 7 fps (0.7%).... Typically SSGs have a narrower ES when the preload is set so that the gap is small (0.020-0.060"), but I didn't bother reducing the preload before shooting these strings, so the gap at 6T out was actually over 1/4".... (over 1/3 of the hammer travel wasted)....

I then filled the gun to 2900 psi (my Great White needs filling), and shot an entire string with the gun adjusted for 38 FPE (an average velocity of 972 fps with the 181 gr. JSBs).... I stopped when the first shot dropped below 950 fps, which occurred at shot 45.... Yep, that's a total of over 1700 FPE on a single fill.... Here is the string....



The ES wasn't great, about 2% with unsorted pellets and the large gap in the SSG.... You can see what happens to the pressure curve at the regulator setpoint of 1150 psi, the slope decreases because both tubes are dropping in pressure below that.... I got 35 shots from 2900 psi to the setpoint, another 5 below that down to 1000 psi.... and another 5 shots before the declining velocity opened the ES up to 4%.... The overall efficiency over the 45 shots was 1.44 FPE/CI, down a bit from the results above at 37 FPE.... so I appear to be right in that area where a slight change in gap and velocity makes a big difference in efficiency.... If I were really interested in tuning for these pellets, I would use less preload on the hammer spring and reduce the gap in the SSG to make the gun easier to cock.... I would expect that to narrow the ES and increase the efficiency as well....

This completes my testing of the regulator, now it's time to change out the SSG for a conventional preloaded spring (temporarily) so that I can crank the power up with heavier bullets and see just what this gun is really capable of.... 

Bob

[screwwork]

Nice shot string Bob 
I cannot wait to see the conventional preloaded spring results with the BBT slugs.

[lloyd-ss]

Bob,
Very nice results from your new regulated airgun.  The performance of the gun and regulator seem very good, and like you said, now it is time to work with the SSG. 
I am curious about how many shots you have taken in the past few days.  Even with all your years of data collection, and being able to make a close educated guess for the various initial settings, I bet it is a lot more shots than people might think.  It is easy to burn up a tin of pellets (or a tank of air) when generating data like you do. 

That final 1150psi string with the 18.1 gn pellets is very impressive, and it falls nicely off the reg at the end of the string.
Shooting at only 1150psi, that leaves a LOT of room to adjust the pressure for heavier pellets/bullets.
Thank you for putting the regulator through its paces.

Lloyd

[rsterne]

Just the testing with the 18.1 gr. JSBs burned through more than a tin of pellets.... I have all the pellet tins I have ever used, except the few dozen I have used as targets.... They currently fill three canvas rice sacks.... literally hundreds of tins over the years.... BTW, they make a great target at 100 yards.... if you can hit one 5 out of 5 you're doing OK....   



The larger tins I recycle to hold my cast bullets....

The string actually extends about 150 psi below the 1150 psi setpoint before starting its decline.... I can shoot down to 1000 psi without any loss of velocity, and get 40 shots (4 magazines).... Considering that is at 38 FPE in .22 cal, it certainly shows what the Regulated Disco Double can do.... and I think there is a bit more efficiency left on the table.... Of course since your lower reservoir is safe to well over the 3000 psi I was using, there is at least another magazine of shots available with a 3500 psi fill as well.... Over 2000 FPE total on a fill in .22 cal is quite an achievement, you are to be congratulated.... 

Bob

[rsterne]

Time to get down to business tuning this gun the way I wanted it in the first place.... The SSG using the QB spring did not have quite enough power, so I replaced it with a conventional RVA using the same spring but with preload.... In addition, I added a lightweight sliding spring guide as a valve lift indicator, so that I could make direct measurements of the valve lift at various preload settings.... Here is the SSG (lower) and the Lift Measuring Rod for comparison....



The same spring (from a QB) is used in both, so you can see how much preload I have in the SSG.... Conversely, you can see that with the preloaded setup, the spring will still be pushing the valve open throughout the entire shot cycle.... The Lift Measuring Rod is just a piece of 3/16" K&S aluminum tubing with a short flathead 8-32 screw in the end.... the head of the screw ends up captured between the spring and the hammer, so the tubing slides with the hammer.... There is a #008 O-ring on the end, which you push up against the RVA adjusting screw, then cock and fire the gun.... Since the tube travels with the hammer, the O-ring slides back along the tube the same distance the valve opens.... so the distance between the RVA screw and the O-ring is the same as the valve lift....

The regulator pressure was still sitting at 2000 psi, within as close as I could measure it, so the regulator seems to have settled down nicely and is now showing no signs of creep overnight.... When it was brand new, it was gaining about 100 psi overnight, and it is not unusual to see a short "break-in" period like this.... I filled the lower reservoir to 3000 psi, cranked the preload to maximum (coil bind), loaded the magazine with the new 34 gr. JSB Beasts, and fired 3 shots across the Chrony.... The velocities were 1006, 997, and 997 fps, and it used 1000 psi for the three 75 FPE shots.... Incidently, 75 FPE is exactly what my Spreadsheet predicted for my "lofty goal" with a 24" barrel at 2000 psi.... The barrel on this gun (although I ordered a 25") is 28" long, so that is 100% consistent with my expectations.... to fall short of the spreadsheet prediction by a small amount.... I measured the valve lift, and the O-ring gap was 0.200", which is all the travel the valve stem has before being driven flush with the back of the valve, so I had all the hammer strike I can possibly use (and more than I need).... I turned the RVA out 1 turn, shot another 3 shots, recorded the velocity, pressure drop for 3 shots, and valve lift, and continued doing that until the velocity dropped below 600 fps.... Here are the results....





There is a LOT of useful information in those two charts.... First of all, the velocity plateau, which is at 997 fps, starts when the preload is backed out 6 turns (1/4") from coil bind, and in fact there is no real drop in velocity for the next couple of turns.... At 6 turns out, the pressure dropped 580 psi for 3 shots averaging 994 fps.... but at 8 turns out, the pressure dropped only 400 psi for 3 shots averaging 983 fps.... At 9 turns out, the pressure only dropped 320 psi for 3 shots averaging 963 fps.... In that 3 turns of adjustment, I lost 3% in velocity and used 45% less air.... and the efficiency increased from 0.74 FPE/CI to 1.25 FPE/CI.... Compared to maximum preload, when the hammer was hitting the back of the valve, it is only using 1/3rd of the air.... So 9 turns out is clearly the "knee" of the curve, with the downslope starting at 10 turns out.... You will also notice that until the preload was backed off 2 turns from coil bound, the lift stayed constant at 0.20" because that was the maximum the valve could physically open.... At 6 turns out, where the velocity just starts to drop, the lift is down to 0.14", and at 9 turns out, at the knee of the curve, it is just 0.10".... It is that decrease in lift, and its associated decrease in dwell, that decreases the amount of air released by the valve.... Since the throat of the valve is 0.266", driving the valve more than half that distance open gains nothing in power, it just wastes air.... I have seen this many times before, and now believe it to be a good rule of thumb for how much you need to allow the valve to open.... If it can physically open more than half the throat diameter, that's all you need to allow for.... That means that shortening the valve stem can allow for more useful hammer stroke, which can then reduce the cocking effort.... I may in fact drill a recess in the front of my hammer to allow another 0.050" of hammer stroke.... That should have NO impact on the maximum power, I should still be able to reach the plateau, even with the 34 gr. pellets....

I haven't yet shot a full string with the 34 gr., but based on these tests I should be able to get one 10-shot magazine at 960 fps (70 FPE) by the time the pressure drops to the 2000 psi setpoint.... and that was my goal.... In fact, when I started, I planned on using a PRod magazine, and they are only 8 shots.... but at that time I was unsure if I could get enough power at 2000 psi, and thought I might have to use 2200 for my setpoint.... I did fire a few shots with my 30 gr. BBT, and at 9 turns out the velocity was 1023 fps, dropping to right around 1000 fps at 10 turns out, so logically I should be able to get more than 1 full mag. with the BBTs.... One other thing I should mention, there is a BIG decrease in the report when you hit the knee of the curve, which is to be expected....

Bob

[rsterne]

I milled a 3/16" diameter recess in the front of the hammer that is 0.050" deep to increase the hammer travel to 0.75".... Without changing the preload I gained about 20 fps at the 10T out position.... This is because the cocked force is the same but the uncocked force is less, dropping the average spring force slightly while increasing the travel distance.... This slightly increases the hammer strike without any increase in cocking force.... I shot a 10 shot string at that setting, and it had not dropped below the setpoint, proving the gun is capable of doing that with the 34 gr. JSB Beasts in the mid 900s on a 3000 psi fill....

I then shot a few shots with my 29.6 gr. BBT FN bullets at each preload from 10 to 12 turns out.... It was nearly 1000 fps at 10 turns, and just under 900 fps at 12 turns.... so that determined that 11 to 11.5 turns will be the likely setting for the FN bullets in the mid 900s.... It was about 50 fps faster with the 27.4 gr. HPs, so 12 turns out will be quite close for those.... I also shot some short strings with bullets sized at different diameters at 11 1/4 turns out, and got the following average velocities....

0.216" FN .... 972 fps
0.217" FN .... 963 fps
0.218" FN .... 956 fps
0.219" FN .... 956 fps
0.217" HP .... 1007 fps

I was curious what the relationship would be between diameter and velocity, and as expected the smaller bullets shot faster, because of less resistance inside the barrel.... I wondered about why the "as cast" bullets (they are the 0.219") weren't slower, and then I remembered that the chamber is 0.218", and the bullets were noticeably harder to load than the others.... so it was resizing them to 0.218" before they were shot, hence the reason the velocity stayed the same.... The total pressure drop for 10 shots was around 950 psi, so the pressure stayed above the setpoint for all 10 shots, in fact I would expect the 11th shot to not lose any velocity.... not that it matters, because it is a 10 shot magazine....

Bob

[David Mccann]

sweet build.... great gun...congrats

[Buldawg76]

You have done it again with a very nice build and excellent performance out of various pellets and bullets.


Mike

[rsterne]

I installed the shroud today, and a scope and bipod.... It weighs 8 lbs. 10 oz. complete, or 8 lbs. even without the Bipod.... Without the scope and rings it is about 6 lbs. 12 oz....



I played around with the preload to find a setting that could work for both weights of BBTs and the 34 gr. JSB Beasts.... The RVA ended up at 11.25 turns out from coil bind.... and the gun is quite easy to cock like that.... I then shot some 10-shot strings, refilling to 3000 psi each time.... At the end of the strings it was pretty much right on the 2000 psi setpoint.... so I can shoot one magazine on a fill.... Here is what they look like....



The 34 gr. JSB Beasts averaged 874 fps (57.9 FPE) at an efficiency of 1.35 FPE/CI.... The 29.6 gr. BBT solids averaged 944 fps (58.6 FPE) at 1.24 FPE/CI.... and the 27.4 gr. BBT HPs averaged 988 fps (59.4 FPE) at 1.21 FPE/CI.... This was using BBTs sized to 0.217".... The ES on the Beasts was just 6 fps (0.7%), and with the BBTs is was just under 2%.... That should prove close enough for shooting out to 100 yards....

I then took the opportunity to see what adjusting the barrel tension does to the group size.... Without the shroud, the 28" barrel was pretty whippy.... It shot OK inside my shop, but the distance is only 20 ft.... Still, I can usually tell if the gun is a shooter or hopeless just during my Chrony sessions at that distance, if I pay attention to aiming.... The JSB pellets were pretty much all 5 touching, but the BBTs ranged from that to a loose 5-shot group you could just cover with a dime.... I installed the shroud, but with no tension on the Bellevilles, just enough to keep the tension adjusting nut from falling off the muzzle.... This did, however, drastically raise the POI, because I had indexed the barrel to the bottom, and it had a noticeable curve to it.... Once I rezeroed the scope, and using the 34 gr. JSB Beasts, I shot a series of 5-shot groups, tightening the nut on the Bellevilles 1/4 turn at a time.... I am using a stack of 5 Bellevilles, and they have a rate of 750 lbs. per 0.010" (singly), so that means each 1/4 turn increased the barrel tension by about 200 lbs.... I stopped at 1.5 turns, which would be about 1200 lbs. of tension in the barrel.... In theory I can go another 1/4 turn without driving the Bellevilles flat, but I don't like to do that, you can ruin them by causing them to take a "set".... I disassembled the stack after tensioning to 1200 lbs., and they had only collapsed less than 0.001" each, so I will use that 1.5 turns as my limit.... Here are the group sizes as I increased the tension with the 34 gr. Beasts....

0 lbs. tension = 0.30"
200 lbs. = 0.44"
400 lbs. = 0.22"
600 lbs. = 0.36"
800 lbs. = 0.19"
1000 lbs. = 0.17"
1200 lbs. = 0.28"

I then repeated the process with the 29.6 gr. BBT FN, sized to 0.217"....

400 lbs. tension = 0.38"
600 lbs. = 0.53"
800 lbs. = 0.48"
1000 lbs. = 0.59"
1200 lbs. = 0.15"

I then tried the 27.4 gr. BBT HP's, also sized to 0.217"....

800 lbs. = 0.45"
1000 lbs. = 0.21"
1200 lbs. = 0.34"

and then at that 1000 lbs. setting I tried other diameters of the HPs....

0.216" = 0.32"
0.217" = 0.21"
0.218" = 0.24"
0219" = 0.25"

I then reduced the tension to 1 turn (800 lbs.) and tried the HPs again....

0.216" = 0.22"
0.217" = 0.45'
0.218" = 0.15"

What I had been told was that once you reached a certain barrel tension, tightening past that point would not decrease the group size.... What I found was that low tensions did not work well, and that I do indeed need about a turn on the Bellevilles (about 800 lbs. of tension) as a minimum.... However, past that minimum, each bullet appears to have it's own optimum tension setting (sweet spot).... and that can change when you change the bullet diameter.... It likely also changes with the velocity.... In fact, anything that changes the dynamic vibrations of the barrel will likely move the sweet spot a bit.... The good news is, that the group size is dramatically reduced by using the tensioned barrel.... I would say that is a 100% success....

Incidently, when I started testing the tension adjustment I found that the POI on the target was walking around in a 6" circle, even at just 20 ft., as I changed the tension.... Since I had drilled only a 1/4" hole in the barrel extension that holds the Bellevilles, I suspected that the pellets were clipping the inside of it.... I removed it and couldn't see any marks inside, but I drilled it out to 9/32" (to provide 1/32" clearance for the pellet) and this went away completely.... There was a slight POI change with tension, as you would expect, but we're talking 1/2" instead of over 10 times that much.... The report is less, with the muzzle blast venting into the shroud, it sounds like a gun where you just backed off the preload a couple of turns.... You can start to hear the ping, but it is certainly not "backyard friendly".... In other words, it made a noticeable, and welcome difference, certainly worth doing if you live where they don't frown on having a quieter airgun.... It seems silly to me that adjusting the power to drop the noise is OK, but venting the air into the shroud and getting the same effect isn't.... It certainly is nothing like what you can achieve with baffles, or a separate LDC.... but it will protect your hearing somewhat, and well worth the 3" increase in length of the shroud....

Bob

[lloyd-ss]

Bob,
EXTREMELY interesting data on the tensioning and the accuracy opportunity that it presents.  I think you will agree that a heavy, stiff barrel, will be the most accurate.  But most airgun barrels are not stiff, and instead are usually clamped at one end and cantilevered for their full length.  The perfect scenario for vibration.  Your barrel is tensioned, and basically clamped at the breech, and pinned at the muzzle.  The tensioning changes the natural frequency of the barrel, with various nodes and anti-nodes, but the air impulse again modifies that frequency.  That is shown because each type of pellet has a different sweet spot in the tensioning for the most accurate shooting. When the tension is not correct for the ammo, it appears that the barrel vibration is still overpowering the stiffness of the shroud.  My question is, could a single doughnut shaped snubber or damper (or 2), tight against the shroud and the barrel, strategically placed inside the shroud, change the barrel vibration frequency enough so that a single tension setting might cover all (or almost all) of the different projectiles?  It would be like the barrel is now clamped at the breech, pinned somewhere along its length, and then pinned at the muzzle.  It almost seems like having a damper at the anti-node (point of maximum vibration amplitude) closest to the muzzle might work.  Or maybe it should be at the anti-node closest to the breech.  I don't know.   But no matter what, it should make a difference of some kind.
Lloyd

[rsterne]

Yes, I plan to try an intermediate support.... I wanted to try it without as a benchmark.... I don't know if I will bother testing it inside (ie if I can get accurate data at such close ranges).... but the position that makes sense to me is about 3/7ths of the way along the unsupported length of the barrel, or close to that point.... The shroud is supported by two bands in O-rings.... I may remove the top ring of the rear one because of scope clearance issues, I had to slide my scope ahead to miss it.... The forward band is very close to 3/7ths of the distance from the front of the receiver to the muzzle.... A collar setscrewed to the barrel, with an O-ring on the perimeter that slides inside the shroud just in front of or behind that band (so that I can tighten the setscrew) would seem a logical place....

A free floated barrel has, as it's fundamental frequency, one with a wavelength twice the unsupported barrel length.... ie where there is a node at the receiver and the muzzle whips around (the anti-node).... In addition, it can have secondary, tertiary, and higher harmonics, at multiples of that frequency.... The secondary would have the muzzle as a node, and the mid-point of the barrel moving up and down.... the tertiary would have a node at 2/3 of the way from receiver to muzzle, with the 1/3 point and the muzzle moving up and down.... as the frequency increases, the amplitude decreases, because of the inherent stiffness of the barrel itself.... Free floated barrels are used exclusively on target rifles, but the best of them are short, and large in diameter.... This makes them stiff, which raises the frequency and lowers the amplitude.... Our typical barrels are almost the opposite, they are long and thin.... we need the length to get the velocity, and we use thin barrels to save the weight (and because many of them are barrel liners, designed to go inside a larger OD barrel).... I feel that the biggest problem we face in airgun accuracy is barrel flex and harmonics.... and in particular that fundamental frequency, where the muzzle is whipping around....

Using a shroud in compression, and the barrel in tension, helps to stiffen the barrel, because the compression force, once great enough, presses the shroud against the front of the receiver, and stiffens that joint.... An even better solution would be to eliminate that joint altogether, which is why I designed my Monocoque the way I did.... The shroud extends back and surrounds the receiver, eliminating flex in the exterior structure because there is no joint there.... Using a long stub on the receiver, that is a close fit inside the shroud, like I did on this gun certainly helps, but I think you need quite a bit of compression before that joint ceases to move.... I think that is what you are seeing when the group size tightens up at around 800 lbs. of tension/compression.... The shroud starts working like an extension of the receiver.... At that point, the effective barrel OD becomes the shroud OD, which stabilzes the muzzle to a great degree....

At that point and beyond I think the barrel is acting like a guitar string inside the shroud.... as you increase the tension, the frequency increases, and the primary frequency is now where the middle of the barrel is moving up and down, with a few harmonics thrown in.... If you place an intermediate support inside the shroud, you will create a node at that point.... If it is in the middle, you will emphacize the 2nd, 4th, 6th, and any even number harmonic.... So, it makes sense to me to choose an odd number.... If you use 1/3 distance, you will also have harmonics at the 6th and 9th harmonics, so the logical choice to me is either 2/5ths or 3/7ths of the way from one end.... staying with the prime numbers.... It probably doesn't matter which you choose, so I went with the higher frequency one, hoping to force the barrel to vibrate at the 7th harmonic, with 3 nodes behind the support, and 4 in front.... The idea is that the amplitude of the vibrations, because of the stiffness of the barrel, becomes miniscule, within the shroud itself, making the barrel act even more as if it was a solid piece, the diameter of the shroud OD, which in this case is 7/8"..... but at a fraction of the weight....

That's the theory, anyway.... Tom Costin found on the Slayer, which uses a tensioned barrel, that as he increased tension he could actually hear the barrel vibrating inside the shroud, with the frequency increasing with tension.... My gun, without a moderator or baffles is too loud for that, but I have no doubt that note would be there.... Tom uses an intermediate support, "near but not at" the middle, and told me that eliminated the "twang" of the barrel.... My guess is that if he is near the 7th harmonic, it just moved the frequency up above our range of hearing, and decreased the amplitude as well.... 2/5ths is 40%, and 3/7ths is 42.8%, and 4/9ths is 44.4%, so I think anywhere between 40-45% should be a good place for an intermediate support.... It should place the node far enough away from the 1/3 and 1/2 points to avoid those (larger amplitude) harmonics....

Bob

[lloyd-ss]

Very good Bob, I see you have done your homework well.
Lloyd

[rsterne]

In theory.... 

Bob

[Sfttailrdr46]

I don't always understand exactly what you are doing on the first read but this time I can agree that theory is awesome stuff until it jumps up in your face and tells you to rethink the theory 

[rsterne]

I made a few small changes to the gun today.... I removed the top portion of the rear triple band because it interfered with the scope position.... It now only clamps the two reservoirs together.... The forward band remains the same, with an O-ring supporting the shroud, so that it can move independently of the reservoirs for expansion, but is located to prevent damage if you drop the gun or fall on it.... I call this arrangement "semi-floated".... I added an internal support, just a simple collar that is setscrewed to the barrel, with an O-ring in a groove that is in firm contact with the inside of the shroud, flush with the front of the band that supports the shroud.... That places it 3/7ths of the distance from the receiver to the muzzle, to dampen any harmonics between the tensioned barrel and the shroud.... I also machined twelve marks on the tension adjusting nut, to make a Vernier so that it is easy to ascertain the position of it during tuning the tension.... Every 3rd mark is slightly longer (3, 6, and 9 o'clock), and the 12 o'clock mark is the longest, and marked with black.... There is a corresponding mark on the top of the flange on the CF shroud that doesn't rotate as a reference mark....



It is the one that was vertical when the nut just touched the Belleville washers, and when tightened one turn puts nearly 800 lbs. of tension in the barrel.... I can tighten at least another half turn from there, and I can record the position as "3 o'clock", or "4:30", or "5:20".... which will make it easy to return to the setting which worked the best.... Each "hour" of rotation is about 65 lbs. of barrel tension.... I haven't yet tested the gun with the intermediate barrel support, and the Motel is booked until after New Years, so it may be a few days before I get the chance.... and even then it will only be at close range inside my shop.... I am hoping the intermediate support will even out the differences between pellets, and allow one tension setting to work for all.... I guess we'll see what happens....

Bob