.30 cal Disco Double

[rsterne]

I finally got around to fitting my new .357 cal barrel this week.... Sean Pero and I designed a new airgun specific barrel to run anything from roundball to about 160 gr. bullets.... It is 0.357" groove, 0.350" land, with a 26" twist, with 6 narrow lands, and is hammer forged in CrMoly with a 1/2" OD.... The one I got is 28" long.... I had a chambering reamer I had made for my LW 9mm barrel, and it worked just fine.... I made the chamber long enough to work with the 132 gr. Lee RF bullets I had, but in fact even the 154 gr. will chamber without too much effort.... When you chamber a JSB .35 cal Exact, the head is most of the way engaged into the lead and the skirt stops just short of the beginning of the leade, which starts about 1/10" past the barrel port.... The bolt is sealed with a 9mm x 11mm x 1mm O-ring in a groove in the barrel.... It's a very skinny O-ring but seals well and so far seems to be standing up just fine.... The only thing that is a no-no is pushing the pellet out backwards into the breech, the skirt cuts the O-ring.... I found that out when I was checking what the rifling engagement was like....

The valve in the gun is the same PEEK valve I recently made for it, and it has a 1/4" throat and 0.219" ports, which are on the small side for this caliber, so I really didn't know what to expect for power.... The gun was still set up for the .300 cal I recently tested, so the first couple of shots were at that setting, and went across the Chrony at about 920 fps, which was a pleasant surprise, at nearly 150 FPE.... I topped up my SCUBA tank during dinner and this evening got down to some serious testing.... I filled the gun to 3000 psi, cranked the hammer spring preload to just shy of coil bind, and tested all the various 9mm/.357 ammo I have.... Here are the results.....

68 gr. Roundball: 999 fps (149 FPE)
78 gr. JSB Exact: 961 fps (159 FPE
90 gr. Air Venturi HP: 925 fps (171 FPE)
117 gr. Air Venturi RN: 848 fps (187 FPE)
126 gr. Air Venturi RN: 817 fps (187 FPE)
132 gr. Lee RF: 808 fps (192 FPE)
154 gr. Lee RN: 742 fps (188 FPE)

In every case, the second shot was a lot slower, and 2 shots dropped the pressure to 2260 psi, which works out to an ear-splitting 0.40 FPE/CI for (non)efficiency.... Well, the gun was a bear to cock like that, but at least I had enough hammer strike to get everything out of the restricted porting that was possible.... It was pretty apparent that with the porting the gun has, the 78 gr. JSB pellets, which were the original design intention for this gun, were just about perfect.... so the rest of the testing was done with them.... I shot strings at gradually reducing hammer spring preload, recording all shots with 4% of the peak velocity for that string.... Here are the results....



At Max. preload the second shot was down more than 4%, and at 1 turn out the velocity was only down 2 fps, but the gun did manage a 2nd shot within 4% ES.... The velocity was obvious plateaued at the 3000 psi fill pressure, confirming that I had enough hammer spring to get everything out of the gun.... It got 3 shots at 2 turns out, and 4 shots at 3 turns out, with the first two nearly identical, and at that point the efficiency had improved to a still dismal 0.65 FPE/CI.... At 4 turns out the spring has no preload but also no slack when the gun is not cocked, and the gun was now showing signs of a normal shot string, delivering 6 shots with 4% ES at 0.78 FPE/CI.... At 5 turns out, the velocity had dropped quite a bit, and the first two shots were more than 4% below the peak, but the string had 9 shots with a 4% ES, starting from 2800 psi, at an efficiency of 0.96 FPE/CI, not bad for a big-bore.... I turned the preload back in 1/2 turn for the final test, and got 7 shots with 4% ES, the first 6 were within just 17 fps (2% ES) at an efficiency of 0.85 FPE/CI, with the string ending at 1940 psi.... The 7 shots averaged 884 fps (135 FPE), which I'm pretty darned happy with....

This barrel was kind of an afterthought for this gun, the porting was optimized for .25 cal bullets and .30 cal pellets, weighing about 50 gr.... I'm pretty sure that with larger porting the efficiency would increase significantly, and it would probably be possible to tune for a string in the mid 900s with the JSB pellets (ie 150-160 FPE) with decent efficiency.... Anyway, when you consider the gun started out as a humble Disco, and it's still using a stock valve with the ports just opened up to 0.219" and a PEEK poppet.... I think it's pretty impressive to get 6 times the original FPE with a usuable 7 shot string....

Bob

[Doc Sharptail]

Hi Bob:

It would be interesting to see what you get when you dial back a bit~ especially with the pistol design bullets... can't say much for the current design Lee's, but back in the day, the older versions seemed to work well out of p.b. guns at around the 680-750 fps window. It was possible to drive them faster with an adjustment of the powder measure, but accuracy dropped like a stone above 750...

Keep us posted!

Regards,

Doc Sharptail

[rsterne]

I put the .300 cal barrel back on today because I had more testing to complete.... I never worked out a useable tune with the PEEK valve, new hammer spring, and the 70 gr. Bob's Boattails.... So, here is the data, along with that for the JSB 50 gr. (Daystate) pellets....



In a similar state of tune, the heavier, cast bullets produce about 20 FPE more energy than the pellets, but at about 80 fps lower velocity.... To do that, they require one more turn of preload on the hammer spring to compensate for the greater mass.... The shot strings are shorter because you are extracting more energy per shot from almost exactly the same pressure drop.... In both cases, for the "black" strings, the end pressure was just over 1800 psi, starting from 3000, which is a fairly wide range to stay within a 4% ES.... That seems to be a characteristic of the PEEK valve, extending the useful pressure range, and hence shot count.... Here are the tabulated results:

50 gr. JSB pellets:
7 shots @ 100 FPE @ 0.93 FPE/CI
13 shots @ 94 FPE @ 1.02 FPE/CI
14 shots @ 86 FPE @ 1.01 FPE/CI

70 gr. Bob's Boattails:
4 shots @ 131 FPE @ 0.51 FPE/CI
6 shots @ 119 FPE @ 0.76 FPE/CI
9 shots @ 110 FPE @ 0.85 FPE/CI
11 shots @ 104 FPE @ 0.93 FPE/CI

The valve in this gun, which has a 1/4" throat, is plenty for the 90-100 FPE range for the JSB pellets, but it's struggling a bit with the 70 gr. boattails, just as it is with the .357 cal barrel.... It simply can't flow enough air to develop over 100 FPE with good efficiency.... I understand that Sean is planning to fix that by sending me a new valve to play with.... My problem will be to figure out how to make the transfer port large enough to handle the extra flow....

Bob

[rsterne]

After a busy Summer in the Motel, I finally got a chance to get back into the shop.... I dragged out the Disco Double, which had been leaking, and pulled the valve because I knew all the other O-rings weren't leaking.... While it was out, I drilled out the throat from 0.250" to 0.266" to make it about 10% larger than the rest of the ports.... While that doesn't tend to increase the power, it does tend to increase the efficiency, and that small change I could do without disturbing the OD of the seat, so that the valve wouldn't be any harder to open at 3000 psi.... I put new O-rings on the valve, reassembled the gun, and that indeed was the source of the leak.... I tested all the barrels, and there was basically no increase in velocity, but the efficiency went up 5-10%, so the experiment was not only worth it, but confirmed my previous experiments with slightly oversized throats.... The exhaust, transfer, and barrel ports are 0.219", and the equivalent throat area (ID minus stem) is 0.241", which is 10% oversize in diameter, and about 20% in area....

During the testing of each caliber I used one pellet and one bullet.... In .250 cal I used the 25 gr. JSB King and my 51 gr. Bob's Boattail.... For the .300 cal I used the Daystate 50 gr. pellet and my 70 gr. BBT.... In the .357 cal I used the JSB 78 gr. pellet and my 127 gr. Boattail.... I tried different preload setting with each until I found the one that would give me a starting velocity just within 4% of the peak for that pellet/bullet.... When I was finished, I decided to put my "Best Tune" picks all on one graph, so that we could see how the different calibers compare, and here it is....



The dashed lines are pellets, and the solid lines are bullets.... black for .25 cal, blue for .30, and red for .35.... All strings except the .25 cal with the JSB Kings (and the .357 with bullets) were shot with the barrel port wide open, and the hammer spring preload adjusted so that the first shot at 3000 psi was just within a 4% ES of the peak for that string.... Therefore, they represent the most powerful and longest string I can get with a 4% ES starting at 3000 psi.... Had I done that for the .25 cal Kings, the velocities would have been approaching 1100 fps (the gun can drive a King at over 1200 fps, WFO), so I cranked in the adjuster that prevents the flat nosed (probeless) bolt from being withdrawn clear of the transfer port a full 5 turns (7 is fully closed).... That dropped the velocity into the 950-1000 fps range, and I know that the Kings are still accurate there, so it seemed like a good choice.... and easy to change my mind on the velocity by small adjustments on the bolt restrictor.... I ended up with 26 shots (over 3 magazines) at over 55 FPE average from the 250 cc reservoir, which is over 1.20 FPE/CI, stellar efficiency at that power, IMO.... The other exception is the 127 gr. bullet in .357 cal. where I got 3 shots within a 3% ES, without regard to the efficiency, which turned out to be 0.62 FPE/CI.... not bad for 170 FPE....

The other 4 strings all ran from 0.91 to 1.06 FPE/CI.... I should mention that if I want to tune those 13-14 shot strings for a single magazine with the minimum possible ES, by increasing the preload about 1 turn I can get less than a 2% ES for 1 mag. at significantly more FPE.... This is always an option to trade shot count for more power and a tighter ES if necessary.... Of course if you want to tune for slightly less power by turning in the bolt restrictor slightly, then those 13-14 shot strings can easily be stretched to two full magazines as well.... Anyway, all in all, I'm very pleased with the final results.... The gun turned out to be good with pellets and bullets in all three calibers.... It spans the range in power from Ground Squirrels to Deer and Hogs, and everything in between, depending on choice of caliber and ammo.... Pretty good for what started life as a Disco....

Bob