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Discussion Starter · #1 ·
My rifle at a game yesterday: 380 fps with .2's, ridiculously inconsistent (+/- 10 fps). 415 with .36's, ridiculously consistent (+/- 1 fps). So you don't have to calculate, that's roughly 561 fps with .2's equivalent.

I know that heavier bbs allow for more air pressure to build up in a bolt-action, actually increasing fps at heavier weights. I just didn't expect to see this drastic of a difference. Anyone have any insight, especially as to why the consistency changed so much
 

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[quote="zulu":39o5an1i]My rifle at a game yesterday: 380 fps with .2's, ridiculously inconsistent (+/- 10 fps). 415 with .36's, ridiculously consistent (+/- 1 fps). So you don't have to calculate, that's roughly 561 fps with .2's equivalent.

I know that heavier bbs allow for more air pressure to build up in a bolt-action, actually increasing fps at heavier weights. I just didn't expect to see this drastic of a difference. Anyone have any insight, especially as to why the consistency changed so much
 

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Discussion Starter · #4 ·
Gotcha. That inconsistency with the .2's was my main question...I didn't consider BB quality (Doh!).

For everything else it looks like I was making the right assumptions, then. And yeah, if it's shooting not only slightly faster, but with a heavier weight BB
 

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Discussion Starter · #7 ·
See that's why I was wondering how Vindi got that kind of result with his GBB...care to share?


Also, I'm not sure why the fps is THAT low with 2's. I know with the same spring (I hadn't done some compression mods, but...) a few months ago I was getting 540, and it WAS consistent. Fluke?
 

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Its easy, the heavier BB is left in the barrel for longer and as propelant gas relys on acold gas and its inevitable expansion, it gives the gas more time to reach it full capasity for expansion thus forcing the heavier BB out much faster, as with a lighter BB its already left the barrel and the gas is still expanding.

It could be just a fluke of your rifle, but also be aware that on a VSR there is a peculiarity in that the mre hop you apply the more the fps goes up, I think the best results I've achieved on mine was +40fps at maximum hop(not that you'd ever use it like that)

The fps thing can happen because of differing size in BB giving more barrel fill etc in direct regard to the bore size of your barrel, the joule increase is an natural by product of udseing heavy ammo and it happens regardless of which gun type is used.
 

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To be honest no one has nailed down the exact physics of it all as yet.

I'd wager it has

After all theres half the volume of air again in a BA over a AEG, More air = more motive force.

The down side is it will probably deminish your accuracy as its been widly proved that volume matching directly increase consistency and as a result accuracy

And as we all know pure fps does not a sniper make ;)
 

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Discussion Starter · #11 ·
Indeed. I'd probably do some sort of modification for air release (drilling out the slit on the cylinder, or adding a hole at the appropriate length) to match barrel and cylinder volumes
 

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If/when you do it the ratio to start at is about 1.7 to 1 cylinder to barrel, but if you use realy heavy stuff I'd start at 1.9 to 1 as then you can always make it smaller if it needs it.

I always use a 1mm drill and put 4 or 5 holes in, as I always feel a large slot could be damaging the O ring in some way, Just make sure you get rid of all the burrs inside and then polish it like mad to get rid of any scratches.

Balancing a BA can be a pain in the arse cuz you can only get at it from 1 end and everything is done just out of your longest fingers length :-/
 

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how would barrel length in BAs affect this. The bb would have to travel further thus giving more time for air to build up? so in theory longer barrel=more fps. Anyway to know how to calculate a peak for this besides just testing two different barrel lengths? aka how long is too long?
 

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Discussion Starter · #14 ·
I don't understand the exact mechanics of it (and as vindi mentioned, we're not sure anyone does), but I don't think it has to do with barrel length so much as that the cylinder:barrel volume ratio is so much higher on BA's.

Also, there are better ways to achieve fps without sacrificing consistency. This thread was simply meant to illustrate that just because your BA may not be chrono'ing at the power the spring is rated at, this can be drastically affected by BB weight.
 
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