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I recently modified an air cocking sniper rifle. Usually I'm used to messing around with AEG's and there the rule of thumb is that the heavier the ammo, the the lower the FPS. With the sniper rifle, it's the exact opposite.

with .20 gram BB's we got a speed of around 500 FPS. With heavier BB's this increased up to 605 FPS with .32 BB's. Those were the heaviest BB's we had laying around.

Now I'm pondering why this is. The only theory is that the piston produces so much pressure that it is able to propel even the heaviest weight BB up to its maximum velocity. Heavier projectiles have a higher theoretical maximum velocity with enough pushing power.

Is this a correct assumption or does anyone have any other suggestions why the speed of the gun increases with heavier ammo?
 

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Your more or less correct

Its to do with the BB staying in the barrel longer so that the pressure of the piston has time to impart its forces on it for longer. Theres more physics and maths to it than that, but thats basicaly the idea :) now combine this with the ideal length of barrel ( and no, longer is not always better) for the ammo your useing, and you will find that you will probably lose fps if you need to make it shorter, which you probably will..... but the joules will go up even higher ;) just another way of gaining maximum efficiency from your weapon, that doesn't involve making things tighter, bigger, longer, fatter...... ;)

Just be aware that pure fps in no way makes your gun more accurate, it just, like you have been lucky in finding out, enables you to use heavier ammo more effectivley..... which, to a degree, does ;)

All this is especially true on gas guns of any type aswell ;) I gained over 100 fps just by increasing the barrel by 3" and using .3s in my beretta M9, from using a stock barrel and .2s
accurate as hell too
 
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