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
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