BAM B-51 - Better Late Than Never

[rsterne]

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

[Ribbonstone]

Looking forward to what combination you come up with…suspect it will be a different one than most.

Made me read over my notes. Remembering my first impressions of the BAM 51.

1. That’s one heavy spring/cocking stroke. No wonder they put that odd looking cocking piece on the rifle, it took a good firm grip and a firm pull to get the SOB cocked.

2. Why do we have a +3oz striker assembly?  My first deduction was that the valve stem seal was a wide one.  Larger area= more holding force from air pressure = harder hit to get it to open.  Don’t know if I was wrong about thator not as I  went with less air pressure rather than changing the valve stem seal.

3. Not enough difference in energy when cranked “up” to make up for the shot count when slightly cranked “down”.  In your tests, believe  26 foot pounds  X 28 shots is a lot better than 28 foot pounds X 17 shots (basically  728 foot pounds out of a tank full vs. 476 foot pounds). 

Hat trend doesn’t continue forever. Someplace about 23-25 foot pounds it seems to reverse itself (getting less energy per tank full than at higher energy).  This also points to a pretty large air passage from valve to pellet.

4. That air tube really seems thick walled! Guess I got use to the thin tubed PCP’s and thought the BAM’s tube unusually thick (thick enough that threading the actual air tube for the breech screws works fine).

5. Really suspicious of the small diameter bolt and the bolt locking screw diameter.  Haven’t had a problem, but seems like that bolt locking screw really doesn’t leave a whole lot of the bolt diameter. Hasn’t given a bit of trouble, it just looks weak.

Have no idea what the gas tube contributes to the overall weifght,but  have to expect an all wood and metal rifle to have a bit of weight.

6.  Why is the trigger so curled and in the middle of the trigger guard?  Should be a bit less curled and more towards the rear of the guard-space for a better visual (and a less long reach to the trigger).

It’s a minor thing, but did have to get that trigger a bit straighter and working more towards the rear of the trigger than the middle.

7. Whatever that device is on the muzzle, an LDC it is not.  The barrel diameter isn’t a real common one (would be if more rifles were built in imperial measure than in metrics).

[SpiralGroove]

Hey Bob,
Nothing like a new project to lift your Holiday spirits
I have a stock B50 (.22 cal) & B51 (.177) that need some tinkering, it will be very interesting watching the "professor" in his workshop.  It will be fun seeing what a great, unregulated (.22 cal.) string could look like.
If you went with a regulator, I think you should be able to get 40 shots at 875 fps using 15.99 JSB's; 45 shots at 850 fps using the same JSB's would be something! 

Cheers,
Kirk

[rsterne]

The valve throat may be large, I haven't got in there yet, but the ports are tiny.... I assume sized for a .177.... The transfer and barrel ports are under 3mm.... On a .22 cal I would normally use just over 4mm.... It looks like there is enough meat in the brass transfer port to drill it to 4mm (5/32").... barely.... and likewise the barrel port.... but no point in doing that until I pull the valve and have a look at the ports in there.... Hogging out the ports is the route to more power, but usually at the expense of a shorter, peakier power curve.... I agree the hooked  trigger is a disaster, especially for my big fingers.... I plan to anneal it and straighten it some.... That bolt looks weak where the cocking pin goes through it.... one of the reasons I want to reduce the spring load, using more hammer travel to get the lost hammer energy back....

Bob

[doninva]

If you think the gun was dirty, pull the end cap off and look in the air tube. I stripped all three of mine , for regulator install, and found the air tube to be absolutely nasty. Tried to wash tube in parts washer and the crud just laughed at me. Cut a slot in a piece of 5/8" 7075 rod and used 240 paper saturated with parts cleaning liquid, in battery drill, to get it all out and then polished with 400. Can't believe they don't clean them after machining.

[doninva]

I gently straightened my triggers in a vise, with brass jaws. They straightened very easy without any heat.

[rsterne]

With no changes, it looks like the gun takes about 2300-2400 psi to get 875 fps with the 15.9 gr. JSBs.... That's 27 FPE, so we need 27cc for the plenum, plus the volume taken up by the regulator (maybe 20cc?).... That would only leave 150cc (9.1 CI) of reservoir remaining, and from 2900 psi down to 2400 is only 34.5 bar, so you only have 314 CI of air to play with.... If you could get the same 1.20 FPE/CI, that is only 377 FPE, which works out to just 14 shots.... I would go on record as saying there is no way you can get 40 shots at that power with the porting the gun has now.... if ever....

Bob

[Ribbonstone]

You have a “carp load” of posts to look though, and what I’ve found is not  new news, but will list it anyway (just to get your mind going)

The only things I learned about the trigger:

1. The trigger housing is held onto the air tube by two little screws.  If it gets loose, your trigger will either be variable or just refuse to hold.

You can ”cheat” the sear by manually flipping it and get all the internal air tube “guts” out without taking the trigger apart. Can take the trigger apart without having to take the air tube guts out. 

SO the trigger housing has no real need to be detachable. Can get the “guts” out of either one (air tube or trigger unit) and leave the housing in place. 

So I more permanently attached the housing to avoid those screws lossening and screwing up the trigger pull.  Could tack weld it for all the difference it would make, but silver solder, epoxy, or red lock tite will work.

2. The design supplies at least a little sear drag.  The sear itself is spring loaded ”flip” sear, and will drag a little on the striker no matter what you do.  Making sure that the striker is polished smooth and the sear spring is no stronger than it has to be seems to help.

4. Is a bit of a &^^& shoot getting the cat’s tail out of the trigger.  Is possible to break it; being brass makes it tend towards cracking. Annelaing (and as you know, brass anneals the opposite of steel) and bending will work better than just bending.



5. By changing the settings of the trigger adjustments, can get the trigger to sit a bit more back in the trigger guard. This almost always makes the manual safety inoperable.  Simple system, and if you just have to have it, can make the safety work.

6. If you give up on the manual safety (and most will as it seems to work bass-ackwards for right handed shooters) the threaded hole for the safety makes a dandy threaded hole for a spring adjuster (or even reaching in for the stroke adjustment if the back of that “striker nose” screw is slotted and the adjuster hollow).

SUSPCIOUS about the two main faults reported over the years:

1. Suspect leaks are due to the age of the o-rings.  Last ones were imported awhile ago, and no telling how long before that they sat around.  The o-rings are getting old even if the gun hasn’t been used.  I kind of expect one to shrink/get hard/crack by now.  It doesn’t seem to be a design flaw, just old o-rings (that probably started off life sub standard).


2. The valve stem seal has been know to break.  It has a  sharp edge at the reduction for the return spring, and sharp edges focus stress.

The return spring is both stiff and likely to become coil bound if the valve stem is depressed very far.

So am going to guess the combination of a sharp edge at the rebated section of the seal, a  coil bound return spring,  a major-weight striker weight, a strong striker spring, and shooting at too LOW a pressure are the main causes of valve stem seal failure.

[doninva]

I think Ribbonstone is correct on the O-rings. I had to replace  the O-rings on the fill probe on all three rifles. They were rather hard and broke when removing. I bought a pack of each size o-ring for the gun and have replaced them all, some more than once as the regulator and valve has been pulled several times. As stated it would probably be better to anneal the brass triggers before bend but I did three, being very gentle and had no issues.

[SpiralGroove]

Hey Bob,
That sure was a pipe dream -> wasn't it
With your current tinkering, do you have the data to make a chart graphing Velocity (Y axis) and PSI on the X?  This would be using 14.3 or 15.99 JSB's.  This would inform folks like me, what possible setpoints might be picked when using a regulator!

If not, I will do it and post !!!

[rsterne]

Ribbonstone I'm aware of the O-ring and poppet problems, I'll be looking for them when I pull the valve, also the brass filter.... plus a major clean on the tube.... Hopefully I should know enough by then to decide what to do with the valve ports, if anything, when I'm in there.... I already gave the trigger a little squeeze (took a chance and didn't heat it) will try it again to see if I can live with it before annealing it if I decide to have another go....

Unless I'm being fussy for some reason I usually only take pressure readings at the beginning and end of the string.... You can assume the pressure drop is a straight line, and in a regulated gun it should be.... but unregulated it is usually a slight curve, using less psi per shot at high pressure and more at low pressure.... It's pretty easy to plot velocity vs. pressure, I may do that later if I get bored....

Bob

[Ribbonstone]

Will be interested in what you find and what direction you go with that rifle. 

[rsterne]

SpiralGroove.... Here is the plot of Velocity vs. Pressure for the 15.9 gr. Exacts....



I added a trendline joining the lowest velocity and highest velocity recorded and the (estimated) pressures they occurred at.... basically a trendline for all the curves, so I would use that.... Note that 875 fps occurs at 2400 psi.... This is actually a very informative way to look at the data.... particularly if you want to make a decision on a regulator setpoint.... Note that it basically takes hammer strike out of the picture....

Bob

[rsterne]

In this photo you can see the stock hammer, guide, spring, and end cap.... Notice that the striker is sticking out about 2 turns from the nose of the hammer (stock setting was 0.75mm), and also notice the distance from the front of the hammer to the lock screw....



I shortened the front of the hammer by 2mm and made a flush striker to increase the hammer throw by 2.75mm (0.108") over stock.... I also made a very light plastic spring guide that adds only 1/4" of preload to the spring instead of 1/2".... Here are those parts....



I also removed the screw that prevents the hammer from rotating.... It is only necessary if you want to adjust the striker (I don't) and with it removed the hammer can be withdrawn out the back of the tube by lifting up the trigger sear without having to remove the breech.... I reassembled the gun, hoping that the extra hammer travel would mean it would still work about the same with less preload.... It didn't.... I had to put 4 washers under the back end of the hammer spring, and shorten the spring guide because it hit them.... I ended up making the guide the same length as the guide portion on the original metal one because I ended up needing more than stock preload with the much lighter guide.... In other words, I lost more hammer strike by reducing the weight than I gained by increasing the travel.... I shot strings with both 4 and 5 washers, and then put back the original metal guide with no washers, so I had stock preload minus 2.75mm but the spring was still at the same length when cocked as a stock gun.... This combination actually increased the power over stock.... The shot strings, using 15.9 gr. JSB Exacts, are shown below....



The interesting thing is that I gained in efficiency and shot count with the additional hammer travel.... In addition, I ended up with more power when I went back to the stock metal guide (top hat) but with the extra 2.75mm of hammer travel.... Strangely, I also ended up with a bell-curve with that setup, even though the hammer strike was greater than before (when I didn't have one).... Here is a summary of the three strings....

Light Guide, 4 washers.... normal bell-curve, 856-892-856, averaging 878 fps (27.2 FPE), 32 shots at 1.41 FPE/CI, 2900 psi down to 2140....
Light Guide, 5 washers.... normal bell-curve, 870-905-871, averaging 889 fps (27.9 FPE), 28 shots at 1.29 FPE/CI, 3000 psi down to 2270....
Heavy Guide, stock preload.... bell-curve missing some shots at the beginning, 903-924-887, averaging 908 fps (29.1 FPE), 20 shots at 1.27 FPE/CI, 3000 psi down to 2420....

The first string is directly comparable to the first tests at 2 turns out, same power (27.2 FPE), but now 32 shots instead of 25, 1.41 FPE/CI instead of 1.20.... It looks like the additional hammer travel is worthwhile.... I need to fit a preload adjuster to the rear end cap and do some more tests, so that is next....

Bob

[SpiralGroove]

Wow Bob, 1.41 FPE/CI looks pretty amazing
Just tested my BAM 50 (.22 cal.) and it repeatedly get 850+ fps using 15.99 JSB's at 1450 psi (100 bar).
Hence, given Lloyd's calculator, I should be able to get 50 shots at 850 fps going from 200 bar to 100 bar using the same pellet.
This of course assumes I can maintain a 1.31 FPE/CI.  So this is a worthy goal for my tune looking at a setpoint of about 1500 psi.

I'm very interested on your efficiency tweaks (porting/preload mods)................Thanks ----->>>Kirk

[rsterne]

SpiralGroove.... You are unlikely to get 1.31 FPE/CI at 1500 psi, don't forget the average pressure of my strings was nearly 2600 psi.... Of course if you back the power down, that will help maintain the efficiency....

Bob

[nervoustrigger]

Doninva, I think the problem with the O-ring on the check valve of fill port is that the gap between the sealing surfaces is too large.  Thus the O-ring extrudes and shreds.  New replacements don't last that long for most people.  A hard 90 durometer should last longer.

My B51 was one of the "new old stock" rifles that Mike got a couple of years ago and, despite getting lots of use, that is the only high pressure O-ring I have had to replace.  The original O-rings even survived a teardown and rebuild.

[SpiralGroove]

Hey Bob,
One last question :
Generally speaking, what is the slope of the efficiency curve moving going from 100 bar to 200 bar (Pressure on X, E on Y)?   
Kirk

[doninva]

nervoustrigger, good info. Next time I order some o-ring, will get some good for abrasion.


 

[nervoustrigger]

Well, just PM me your address.  I have a whole bag and can never use them all.