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Interesting FPS theory


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#1 kwaispro727

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Posted 18 December 2012 - 05:36 PM

I read online that heavier BBs will result in higher FPS for a gas gun (up to a certain extent obviously).    I saw it somewhere relating to the MP7 so maybe it doesn't apply to all guns.  The guys reasoning was since the BB is heavier it takes longer to exit the barrel hence the gas builds up behind it more.  True or false?

#2 Outlaw1995

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Posted 18 December 2012 - 05:39 PM

It may or may not affect FPS, (depending on length of inner barrel, gun used, weight of bbs used, outside temperature, etc) but using a heavier bb will increase the amount of joules each shot has. Therefore increasing the amount of pain inflicted on the body, and therefore someone thinking you have a higher fps gun.

#3 Chuck S

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Posted 18 December 2012 - 07:00 PM

Not according my my tests.  See my data posted yesterday on GBB carbines and SMGs,

Heavier BB =  Lower muzzle velocity.  Higher energy,  Not for pistols.

-- Chuck

#4 bid2630

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Posted 18 December 2012 - 07:10 PM

If it relates to the MP7, then maybe the reference to increased FPS with higher weight BBs was the result of putting in a longer barrel with barrel extension covering the excess length.  Otherwise I think Outlaw and Chuck have it covered.

#5 gcw360

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Posted 18 December 2012 - 07:35 PM

There are A LOT of variables affecting the FPS of a BB exiting a barrel.  No one has mentioned the BB diameter or material used yet (I don't think)  It does indeed affect the velocity.  In the end, what's important is our gun, with your BBs, YMMV.

#6 Chuck S

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Posted 19 December 2012 - 04:25 AM

See my notes from testing my stock MP7A1 in the "Power Creep" thread which contains a "spirited" discussion of the reasons for energy increase in the earlier posts.  Suffice to say that no matter how this happens the effect is real for many but not all GBB guns.

My MP7A1 MV dropped going from 0.20g to 0.28g BBs (average of a string of 10 shots) but muzzle energy increased.  Same for my LM4, but not for my M9PTP, probably due to the short barrel in the pistol.

Heavier bullets/BBs maintain velocity better than lighter but I've no convenient way to determine velocity at (say) 25 meters.

Energy remains constant with AEGs but since a heavier BB will maintain velocity better it should deliver more energy (downrange) to the target.  But that's another discussion!  Something for you physics majors to delve into over Christmas break!  :)

-- Chuck




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