Developing a bstaley Disco

[rsterne]

Yesterday I took my test gun apart again and removed another O-ring from the buffer, so it is now just two #113s of 70 Durometer.... In order to allow the hammer to engage the buffer, I simply wound the setscrew I am using for a striker out 3 turns (0.108") to allow for the missing O-ring.... Note that as you back the striker out CCW below flush, you increase the hammer throw (to where it first contacts the stem) beyond stock but reduce the preload by the same amount.... At 3 turns below flush, the hammer stroke is now 0.69" and the preload is effectively at 3 turns out, even though the spring remains at coil bind when cocked.... After a few shots to determine where the buffer started to engage the hammer face (2 turns below flush) I shot some strings at maxmum preload.... and then at reduced preload on the last string to move the curve back below 2000 psi....



If you look at the top three curves, you get a perfect picture of what happens as the buffer progressively engages the face of the hammer.... The blue line, with the striker recessed 2 turns into the hammer face, shows the buffer not engaged at all.... The red line, at 2.5 turns recessed, show the buffer starting to engage on the 4th shot in the string, and surpressing the velocity in the second half, shortening the string by 2 shots.... The solid green line, at 3 turns below flush, shows the buffer with increasing engagement, and the entire first half of the shot string is missing, it would require filling to over 2000 psi to see it.... or reducing the hammer spring preload to move the curve to the right.... That is what is taking place with the dotted green line, which is still 3 turns below flush on the striker, but has the preload backed out 5 turns to create an entire shot string without filling past 2000 psi....

As it turns out, I now have three shot strings all at around 750 fps with the preload optimized, but with buffers of 2, 3, and 4 O-rings of 70D.... When I look at the data, I find the following:

Four 70D: 16 shots @ 758 fps (18.2 FPE) @ 1.24 FPE/CI (equals 32 shots in full size Disco tube of 135 cc)
Three 70D: 17 shots @ 748 fps (17.8 FPE) @ 1.29 FPE/CI (equals 34 shots)
Two 70D: 15 shots @ 754 fps (18.1 FPE) @ 1.40 FPE/CI (equals 30 shots)

It is dangerous to drawn any firm conclusions from such a statistically small sample, particularly when the average fps and FPE are not identical.... For example, the small decrease in velocity from 758 fps down to 748 fps could account for part of the increase in efficiency from 1.24 to 1.29 FPE/CI, and maybe for the extra shot as well.... However, there seems to be a slight trend towards increased efficiency with the shorter, and therefore stiffer buffer, but maybe at the cost of a lost shot or two.... Statistically speaking, they are pretty much a wash, so you would be hard pressed to make a choice between them.... When you compare the above with what I got by detuning a stock Disco with just an RVA to about the same power, there is again nothing to choose from.... I got 33 shots at 18.6 FPE within 4% ES with an efficiency of 1.36 FPE/CI from 1800 psi down to 1000....

Bob

[rsterne]

Yesterday I got the chance to rip apart my test gun and mod the Disco valve.... I drilled the throat to 0.234" (still the stock 0.156" stem), and bored out the exhaust port with a 5/32" mill on a 20* angle.... By the time I was done, both had an equivalent diameter of 0.174".... I drilled the transfer port to 0.162" (about as big as you can go without danger of the wall kinking), but out of curiousity (and because I only have one .22 cal barrel) I left the barrel port stock at 0.134".... I was therefore quite surprised when the gun made 31 FPE with 14.3 gr. pellets.... I didn't record a shot string at maximum preload because the 2nd shot was the fastest at 988 fps, but I shot strings at 2 through 8 turns out on the RVA and here are the results....



The average power is 28.3, 25.7, 23.6 and 21.0 FPE.... and the efficiency is 1.02, 1.17, 1.19, and 1.12 FPE/CI respectively.... The pressure shown in the above graph is only approximate, just to give you an idea of the pressure range for each string.... the starting pressures are very close.... These tests are only to establish a baseline for the testing using a bstaley buffer consisting of three 70 Durometer #113 O-rings.... I am using a Challenger hammer with a 1/4-28 SHSS with the point ground flat as a striker, a Disco hammer spring, and a Challenger RVA.... The above tests were conducted with the set-screw flush with the front of the hammer, so the stroke was 0.58" which is stock for a Disco....Remember, for a full volume Disco reservoir, the above shot counts would double....

We're really busy in the Motel right now, so I have no idea when I will be able to get to doing the bstaley tests, unfortunately....

Bob

[rsterne]

I finally got a chance to get back to testing the bstaley Disco with the Modded Valve.... I installed a buffer consisting of three 70 durometer #113 O-rings, and started with the striker set Flush with the face of the hammer.... I then turned it in 1 turn CW, at which point the buffer is basically not doing anything.... I shot a string at 2 turns out on the RVA to compare to the unbuffered setup in the last graph.... I then turned the striker out CCW in 1/2 turn increments, and readjusted the hammer spring preload so that the best curve was obtained using a 2000 psi fill.... All shots over 96% of the peak velocity are shown on the graph below....



At 1/2 turn in on the striker, the buffer starts to engage the hammer after the first 2 shots.... Both these strings are lower in efficiency than the unbuffered version.... I was particularly interested in the (purple) string with the striker set flush, as the velocity is almost identical to a stock Disco, averaging 850 fps (23 FPE) with the 14.3 gr. JSB Express pellets used throughout these tests.... The string was 10 shots, the same as what I got in the very first tests with a stock gun, but the efficiency was less, at only 0.98 FPE/CI instead of the 1.05 FPE/CI for a stock Disco.... Despite the fact that we were starting with a gun with much better flow and capable of nearly 100 fps more that stock, the bstaley buffered version fell short of the efficiency of even a stock Disco....

When the striker was withdrawn another 1/2 turn, the shot string increased to 13 shots, but the efficiency was still not as good as previously obtained.... At 1 turn out CCW from flush on the striker, the gun returned 20 shots averaging 709 fps (16 FPE) at an efficiency of 1.36 FPE/CI, which is very similar efficiency to other strings obtained using various buffers at the same velocity.... The bstaley buffer system, at least in a Disco utilizing a 2000 psi fill, really seems to come into it's own when the gun is detuned to 700 fps or below.... Unfortunately, at higher power levels, I was unable to get any improvement in efficiency compared to simple detuning using just an RVA.... It does permit filling to higher pressures than just using an RVA, and in some circumstances that can lead to a couple more shots.... but I was unable to wring any improvement in air usage out of any of the versions I tested unless the power was reduced substantially....

Bob