I was also going to say that lol if you have extra volume in the cylender when you use heavier ammo it allows pressure to buld more behind the bb creating more energy thus giving you "more fps" in realation to lighter bbs.
In gbb youll notice this even more as the gun will use more gas in a cycle and allow the gas longer to build pressure as it expands behind the bb.
If you have a 4/5 ported cylender try throwin a 455 mm inner barrel in youll notice you will have the 415 fps and when u move up in weight youll have a more even decline in fps.
Most m4's can use up to a 430mm inner(3/4 port) some a 460mm 4/5 port)
http://arniesairsoft.co.uk/?filnavn=...andbarrels.htm
And a closer matched barrel to cylender will make more efficent use of the air.
This is somethin in the sniper world with vsr 10 gspec. Itll shoot 100fps lower with its 330mm inner then the same setup on a 500mm inner.
By porting the cylender to make the compression stroke shorter the fps will raise untill you hit the sweet spot then you will notice the same fps as with a long barrel and non ported cylender with all other things the same
Ill see if i can find it and ill paste it here.