BAM 51 ramble (att. Darryl)

BAM 51 ramble (att. Darryl)

Joined: November 17th, 2006, 3:51 am

June 23rd, 2012, 1:39 pm #1

Want to thank Ed for his suggestion, reducing the sear spring did reduce drag on my striker enough to make a difference in shot count. Understand, not talking about the pull weight...didn't change the other springs that control that.

Darryl:

Think you can get to 28X30 (28 foot pounds/30 shots) or at least very close to it. I tend to use 3% of max. speed to set the sweet spot, so if you the peak shot was 950fps (lets say 18gr.), then the velocity variation limit would be 28fps (950X.03 = 28.5).

(Percentages make it tougher on low powered guns...a 600fps gun would have to live inside of 18fps while a 1000fps gun would have 30fps to play in).

Do get tired of taking the gun apart...so don't. Tuning for power and shot count, will worry about accuracy later, so I don't put the gun fully together for testing. Don't bother to mount the scope, the barrel band, or the brake (even putting it in the stock is optional). Need a bit more time setting up a firing cradle to get your chrony readings, but saves time/effort in the long run.

The Book may not have been about the .22 versions, but the directions of change the mods do apply. You may not be able to do X and Y and come out with exactly Z, but you will get the same direction of change.

Afraid I got side tracked at 25 foot pounds (which is more like 28 foot pounds with 21gr. Pellets). And I still only want to run it at 150BAR.

This looks good for short range (tested at 22 yards), but you have to be critical when using short range. The real variation in these 40 shots is about 50 fps. That's OK for 20-30 yard shooting, but really would be better to say inside of half that for longer distances.
[/IMG]

Cut up and put together like this, with group centers marked and joined, the 40 shots start to look more like a shot graph. More like 25 good shots than 40. If trying for some 75 yard groups, would probably try to stay in the spot where the best 15 shots live (targets #4,5,6)..
[/IMG]

HINTS that it may benefit from a new(smaller diameter) valve seal:

The difference in energy between light and heavy pellets is kind of high, Points to a lot of air being moved per shot.

They tend to valve lock with just a small over fill. Valve closing force is the air pressure acting on the area of the valve seal (along with valve seal material and shape). Air pressure is the same as for other PCPS, takes more of a hit to open them up at the shot, so the valve seal area is probably larger.

Problem spots ( first one related to the above):

Skinny valve stem seal...large valve seal area...heavy closing force...strong opening hit. Seems this is why they occasionally just drive the valve stem right though the valve stem seal.

The screw on the left side of the bolt (the one that actually pulls the striker back) works loose. If you let it get wobbly-loose will bugger up the threads.


Last edited by gubb33ps on June 23rd, 2012, 2:45 pm, edited 1 time in total.
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Joined: January 31st, 2011, 7:14 pm

June 24th, 2012, 9:26 pm #2

Yes that bolt screw will work it self loose but if you put just a small amount of lock-tight on it you will not have that problem .
Question for everyone tuning the b50:
What if the pellet that you are tune for isn't the best pellet for the AG, would it not change everything about the tune that you finally got right with the wrong pellet?
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Joined: November 17th, 2006, 3:51 am

June 24th, 2012, 10:04 pm #3

If the pellets produce wild velocity changes to to diameter or weight variations, then they're useless of testing.

If they give good numbers but just shoot less accuately than some others...then reguardless of weight, would be fine to use for setting the pressure level and getting an idea of shot count.

Heavy pellets tend to earn higher energy (not velocity..they are slower, but not enough slower to earn less energy) and often a bit longer shot count, but it happens at the same presssure range.


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Joined: July 24th, 2009, 4:33 pm

June 27th, 2012, 5:44 pm #4

great stuff! I had to put my 50 down for a while to let my brain reboot from all the various combinations of hammer springs, spring guides, transfer ports, etc. I'll get back to it soon enough, but I'm glad I stuck my head in here. I'll keep this thread in mind, especially about the trigger. I'd known before that sear drag can throw things off, but sometimes one forgets when the forest gets too thick. If I were shooting for competition I'd sure set a smaller sweet zone as you suggest. 30 shots as a goal does not take into account a consistent POI, which grows larger with more shots unless it's a very flat shot string with a low overall Es and Sd. Now if I can get 30 shots with an Es of 20 FPS across those 30 shots I'd be good with that. Better for me, I think to reset my goal by asking how many shots I can get within an Es of 20 FPS.
darryl
btw, 3% of max ix quite a decrease from 4% which is what I tend to use, but what the heck. I like your number better, though it seems on the surface a bit harder for me to imagine achieving. but it would make for a flatter shot string. I've been thinking to be a bit more flexible on the 28/30 number. The shot count is still a goal, but I'll be curious to see what FPE I wind up with at that count.

Also I have Big Ed's valve stem installed. It's supposed to be made of sterner stuff.
Last edited by turbodt on June 27th, 2012, 6:23 pm, edited 1 time in total.
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Joined: November 17th, 2006, 3:51 am

June 28th, 2012, 9:48 pm #5

For a couple of years, ran the BAM as a low pressure "Discovery" style gun. Lower spring tension, easy to cock, and a much lighter striker.

Replaced the steel spring guide with a plastic one...which cut weight way-way down, but limited the operating pressure to about 140BAR. It ran quite well, but over a small range of pressure (about 35-40BAR covered the whole sweet spot). Light weight strikers tend to do that..are efficient, but only in a short pressure range.

Anyway...found the results of that old test. Have to remember, this was after months of fiddling around trying to get the balance of everything as right as I could, looking for the best shot count.


Rifle:
[/IMG]

Results with 14.6gr. Pellets:
128BAR to 90 BAR:
824 790fps (805fps average/21 foot pounds)
If I figured this right, would be:

(196CC X38 BAR) / (21 foot pounds X 40shots) = 9.33

Target: 5shots/20yards:
[/IMG]



Results with the lighter 13.4gr. Pellets:
128BAR to 9 0BAR
851 -819 fps (833 average/20.65 foot pounds)

(196CC X 38BAR) / (20.65 X 40shots) = 9.5

Target: 5 shot/20yards:
[/IMG]

Last edited by gubb33ps on June 28th, 2012, 9:50 pm, edited 1 time in total.
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Joined: July 24th, 2009, 4:33 pm

June 28th, 2012, 10:37 pm #6

Yes that bolt screw will work it self loose but if you put just a small amount of lock-tight on it you will not have that problem .
Question for everyone tuning the b50:
What if the pellet that you are tune for isn't the best pellet for the AG, would it not change everything about the tune that you finally got right with the wrong pellet?
in two different airguns the same pellet may perform differently, of course. Then again, some pellets vary from batch to batch enough to cause an inconsistency issue when testing. I know this to be true of the .22 Crosman Ultra Magnums (14.3 grain domes). When testing my B50, I recently depleted one tin, and opened another tin from a recent purchase. These Crosman pellets had varying FPS results across my chronograph. I admit I did not expect to see such poor consistency within one tin, but there it was. Maybe good for sinkers? I'd wanted to save a buck or two because I had been testing with Jumbo heavies, but the budget can't take too much of that.
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Joined: July 24th, 2009, 4:33 pm

June 28th, 2012, 10:42 pm #7

For a couple of years, ran the BAM as a low pressure "Discovery" style gun. Lower spring tension, easy to cock, and a much lighter striker.

Replaced the steel spring guide with a plastic one...which cut weight way-way down, but limited the operating pressure to about 140BAR. It ran quite well, but over a small range of pressure (about 35-40BAR covered the whole sweet spot). Light weight strikers tend to do that..are efficient, but only in a short pressure range.

Anyway...found the results of that old test. Have to remember, this was after months of fiddling around trying to get the balance of everything as right as I could, looking for the best shot count.


Rifle:
[/IMG]

Results with 14.6gr. Pellets:
128BAR to 90 BAR:
824 790fps (805fps average/21 foot pounds)
If I figured this right, would be:

(196CC X38 BAR) / (21 foot pounds X 40shots) = 9.33

Target: 5shots/20yards:
[/IMG]



Results with the lighter 13.4gr. Pellets:
128BAR to 9 0BAR
851 -819 fps (833 average/20.65 foot pounds)

(196CC X 38BAR) / (20.65 X 40shots) = 9.5

Target: 5 shot/20yards:
[/IMG]
Of course you are correct, a lighter striking force will necessitate a lower start fill. Weaker springs, lighter guides and or hammers all conspire to push the start fill down; just not enough kinetic energy from the lighter internals to pop that valve open when filled to higher start-fills.
Oh wait. I just went crazy trying to keep all this straight!
darryl
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Joined: November 17th, 2006, 3:51 am

June 28th, 2012, 11:12 pm #8

in two different airguns the same pellet may perform differently, of course. Then again, some pellets vary from batch to batch enough to cause an inconsistency issue when testing. I know this to be true of the .22 Crosman Ultra Magnums (14.3 grain domes). When testing my B50, I recently depleted one tin, and opened another tin from a recent purchase. These Crosman pellets had varying FPS results across my chronograph. I admit I did not expect to see such poor consistency within one tin, but there it was. Maybe good for sinkers? I'd wanted to save a buck or two because I had been testing with Jumbo heavies, but the budget can't take too much of that.
The next gun may not show the same velocity variation....have seen it happen with pellets again and again. On a guess, they probably won't ever be the best pellet for a barrel, but in some they may calm down enough to be useful for setting pressure levels.

Or not...if not, let your brother-in-law use them.

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Joined: November 17th, 2006, 3:51 am

June 28th, 2012, 11:22 pm #9

Of course you are correct, a lighter striking force will necessitate a lower start fill. Weaker springs, lighter guides and or hammers all conspire to push the start fill down; just not enough kinetic energy from the lighter internals to pop that valve open when filled to higher start-fills.
Oh wait. I just went crazy trying to keep all this straight!
darryl
Still have the parts, and while it is a very heavy rifle for a 20fp power level, the accuracy and shot count might be worth putting them back in.

BUT don't jump on those shot counts, were shooting well past my velocity limits. Are not 40 shot settings. Are more like 30-32 shot settings for the prefered vel. variation.

(as I am the one who refills the gun, why shoot at less than the best the gun can give?).
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Joined: July 24th, 2009, 4:33 pm

June 29th, 2012, 12:11 pm #10

The next gun may not show the same velocity variation....have seen it happen with pellets again and again. On a guess, they probably won't ever be the best pellet for a barrel, but in some they may calm down enough to be useful for setting pressure levels.

Or not...if not, let your brother-in-law use them.
"Or not...if not, let your brother-in-law use them."
LOL!!
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