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Mr Monkey Nuts

Loss of FPS after short Barrel

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Question for the more experienced techy guys, I have recently installed a 250mm 6.03 barrel in my GR16, (originally a 360mm) since then the FPS has dropped from 335 to 280, nearly 50 FPS. is that normal for a shortened barrel?


Compression seems good, if I block the end of the barrel and fire, it causes a lot of resistance, and the hop up seems to be sealed nicely. It has a m100 spring as it was shooting hot at 380 when I bought it, bearings, shimmed and a guarder high torque motor.


So will 11mm barrel make that much of a difference or should I be looking for other issues?



Thanks in advance

Mr Monkey

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110mm/11cm is a good bit of a change.


You need to match the cylinder volume to the inner barrel volume.


What's happening is that not all of the air in the compression zone is used to accelerate the BB because the piston is coming under compression earlier than it should be and the BB is escaping the barrel before it is fully accelerated.


If you want the rough velocity of what you used to have match the volume.


Here's what you do:


Use πr2h to find the volume of the barrel and cylinder. To find the h of the cylinder measure from the port to the top,this is the compression zone. Find the ratio of the two and try get it as close as possible.

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My Gspec is massively over volumed,the cylinder is roughly matched,and even still over volumed for a 430mm barrel.


I do not experience any noticeable accuracy losses with any weight over .25


The only major effect this has is that the Kinetic energy output with .30s is rather high,touching 1.10J (Borderline illegal in Ireland,but we have the 1J limit unlike the UK)


You can keep it as it is and use heavier ammo to counter any accuracy issues that may crop up,it will accelerate slower than lighter rounds so it will have more pressure buildup behind it,so it will hit harder and retain spin from the hop up more efficiently. Use the heaviest weight your hop up unit can lift to your preferable distance. Try get a small sample of .28s or .30s. G&G hop up units are rather good so I'd say those two weights will be ideal.


You can also jam in a stronger spring like Jcheese said to bring it up a bit. A 110 should get it to 310 or so assuming good air seal. For an M120 you'd need to correct the AOE for reliability. Anything under a M120 can be rather reliable without AOE corrected from what I've seen.

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If your cylinder's capacity is larger than the barrel's then the BB is already out of the barrel before the piston head hits the cylinder head. That means it hits it without the cushioning effect of air behind the BB. Normal V2 gearboxes break because of this really easily. Especially if you put a stronger spring to compensate. G&G gearbox is okay but it won't do any good to that either. You can use a V2 fix for that but it would be much better to use a matching cylinder.

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