About 2 years ago I bought a BAM B-51 in .22 cal from sniper when he got a crate of the last of them.... I knew they should be cleaned before shooting them, and I had other projects on the go, so hung it on the wall until "later".... Well, "later" finally arrived this week, so I read "The Book" (which I had previously downloaded), and stripped the barrel and breech off and pulled the hammer out to clean and deburr them and the tube.... The barrel was filthy, and the bolt was very stiff, so I removed the bolt, deburred all the holes and slots in the breech, and cleaned the barrel thoroughly, and gave the breech bore and the barrel a polish with Bore Brite.... The bolt was still stiff and I suspected it was bent, so over to the lathe, chucked the front, and spun it around to watch the handle wobble around nearly 1/16" off center.... No wonder it was stiff.... I spent quite a bit of time straightening it, and that freed it up nicely.... A thin smear of Moly paste and a new O-ring (the original was shredded from the sharp edges I had to remove) and it was ready to reinstall....
I had read about the hammer, and decided that the most suitable mod from "The Book" was to remove the threaded striker, cut a slot in the back end so that it could be adjusted with a long thin screwdriver through the rear plug, and reinstall it, being careful to put it back to the factory length.... I also put a small piece of plastic under the locking screw for the striker, and tightened the locking screw up to compress it against the threads to provide a "brake" so that the striker won't "self-adjust" and yet I can change the setting from the back.... The hammer is VERY heavy, and has a heavy steel spring shim and guide (top hat) inside, with the total weight being 148 grams, of which the top hat is 38.... This is extremely heavy, and is compensated for in the design by having a stiff spring and very short travel.... The normal method of tuning is to screw in the striker from the back (clockwise from the rear) which makes it protrude even further, shortening the stroke even more.... This moves the hammer back, increasing the preload on the spring, and making the cocking stroke shorter but even harder.... I wasn't too sure about the whole concept, but decided that I needed to test it in stock form and make the normal adjustments to get a baseline for further experiments....
I reinstalled the hammer, leaving the safety off so that I could adjust the hammer travel through the end plug with a long thin screwdriver.... The first tests were conducted at the stock travel setting, filling to 1000 psi for a shot, then 1500, then 2000 (velocity increasing), then 2500, 2800, and finally 3000, which resulted in the highest velocity (938 fps) with JSB Express 14.3 gr. pellets.... So, I knew I was in for a falling velocity shot string, at least with the lighter pellets.... I hooked up my Chrony to my Netbook using Chrony Connect, and starter shooting strings.... I then saved them into Excel to create the following graphs....
The initial tests, at stock hammer travel, showed a pretty powerful gun, 27-30 FPE (heavy pellets giving more FPE, as usual), but only 17-19 shots within my normal 4% ES.... The efficiency wasn't bad, considering the way the gun was tuned, from 0.89-0.95 FPE/CI, with the heaviest pellet having the best efficiency.... So far, no surprises, including the excess hammer strike, I expected that from what I had read about the gun, it was tuned for maximum velocity from the factory, not a proper bell-curve.... I removed the stock and cranked the striker in three turns, grabbed the middle-weight JSB 15.9 gr. Exact pellets, and found out I had gone too far.... The velocity at 3000 psi fell to the mid 700s, and took a dozen shots before it got to within 4% of what ended up being the peak at 874 fps.... After recording the string, I turned the striker out a turn, repeated the process, then once more, and the four strings with the 15.9 gr Exacts are all shown on the second graph above.... Here is a summary....
Stock setting.... decreasing string from 909 fps, averaging 894 fps (28.2 FPE), 17 shots at 0.90 FPE/CI, 3000 psi down to 2320....
In 1 turn.... normal bell-curve, 884-909-873, averaging 895 fps (28.3 FPE), 24 shots at 1.19 FPE/CI, 3000 psi down to 2280....
In 2 turns.... normal bell-curve, 862-896-861, averaging 878 fps (27.2 FPE), 25 shots at 1.20 FPE/CI, 2900 psi down to 2180....
In 3 turns.... normal bell-curve, 840-872-842, averaging 861 fps (26.2 FPE), 28 shots at 1.36 FPE/CI, 2700 psi down to 1980....
This confirmed that the factory hammer strike was too much, turning it out a turn gave more power and more shots at greater efficiency.... After that, turning the adjuster in more just dropped the pressure range and power.... The "proper" setting would be about 2 turns out from stock, using the factory rated 200 bar (2900 psi) fill pressure.... My gun was leaking down slowly during testing, so in fact the efficiency is actually better than what I recorded above.... I did NOT like the feel of cocking the gun at 3 turns out on the striker, and it was barely acceptable at 2.... The bolt pulls back a long way before engaging the hammer, and the cocking stroke is short and requires a very hard pull at that setting.... I think I will concentrate on lengthening the hammer stroke, reducing the preload, and reducing the hammer weight for the first round of mods.... The latter two will be done by replacing the top hat with a shorter unit (to reduce the preload) made from Delrin (to lighten it).... I will likely also fit a preload adjuster into the end cap to revert the adjustment to what I consider to be superior to the current setup.... Here is a photo of the gun, with a Leapers 3-9 x 32AO scope installed....
Bob