BAM B-51 - Better Late Than Never

[GarthThomas]

I put a 3/32" transfer port this morning and I'm down to 18fpe the curve seems flatter but I started with too low a fill for the string and I haven't had time for another try but I may be up to 40 shots within 4%. 18fpe seems to be a threshold for using JSB18.1 to their best advantage, but I find the CPHPs are were to deliver some 1/2"groups at 45yds today in spite of the wind. I think I'll extend the threaded striker next. Its nice to be filling to 110bar, my muscles are getting sore from this tear down, I probably pumped 250 times before I realized I'd only finger tighten the fill nipple, I felt pretty stupid but was thankful I didn't need to tear down again.   
Do you think I'm hijacking Bobs post here.

[rsterne]

I would recommend leaving the striker screwed in flush with the front of the hammer.... If you need less power, reduce the preload on the spring by clipping part of a coil at a time or thinning out the shoulder on the top hat.... or smaller transfer port or higher pressure fill....

Bob

[Ribbonstone]

OK…tried another tune. 

NOT in the spirit of careful experimentation, changed THREE THINGS all at once.  Yes, if I were a careful/scientific kind of guy, would have changed each in isolation, but time/day light constraints made me go for all 3 changes at once.

1. Reduced the transfer port from .120” to about  .110-.113”.  Basically a 7/64ths inch drill, which could have wobbled a few 1/1000ths.

2. Increased the string tension by  .08” (the thickness of the washer used on the striker spring).  Cocking force seemed reasonable  (just barely more than if felt like before if I consentrated on it.)

3. Filled to 170 BAR (about 2500psi) to match the increases spring tension.

End result was this:



1. Under 30 foot pounds. Goal was 30+, so I’ll have to speed it up by about 6%.

2. On an educated-guess, if I increase energy by 6%, am likely to decrease shot count by 12%.

3. If #2 holds true, then the 30 foot pound shot count might work out to about 29-30 shots.  30 shots at 30 foot pounds would have been nice, and may be possible, but right now it looks more like  28-29 shots at +30 foot pound will be the out-come.

4. The “improvements” are coming in small increments.

5. Is a little more efficient that it was.

[SpiralGroove]

Nice Ribbonstone,
So that's 2500 psi down to about 1700 psi, with 1.38 efficiency?  Great that you can get 775 fps using a 21 grain pellet with transfer port at .120".  That higher pressure is making a difference.  My (.177) B50 has all ports increased to .1406 (9/64").  If you can get 30 shots at 30 FPE, that's about a 36% increase from your original shot count
I'd say THAT is mostly definitely worth your toil

[rsterne]

Still improving, I look forward to your final tune..... no doubt you will have a nice setup....

Bob

[Ribbonstone]

The thing about a definitive "goal" is that if even 6% off that goal, have to make the adjustment...and am likely to have to adjust things a 2nd or 3rd time to get it closer to (or a few 1/10ths either side) my 30 foot pound goal.  Know that's kind of pedantic of me, but I set out for 30 foot pounds and will phart around until I get it.

[GarthThomas]

I'm down to 5/64" TP and the top hat shoulder from 3/8" to 1/4" and about 14.5fpe avg. for 40 shots within 4%, I have one smaller TP left and a spare MC spring

[moorepower]

Bump. Sorry.

[toine]

Great thread, tons of amazing info. I've updated the reference site I put together to point to this and other recent posts as well.

[Ribbonstone]

Got side tracked by a BAM 50 in .177, so the work on the  .22 51 got put in the “good enough” category.







Same system with either of them, just playing with spring tension/striker weight/transfer port.  Valve is as-issued.

[toine]

Great motivation indeed. So far I've learned at least one very valuable lesson... I need different tools if I'm going to make my own replacement spring guide.