How to decrease FPD deviation?

[TROOPERUSS]

Speaking in terms of minimizing the variablility between shots, by decreasing the FPS deviation between shots.
What upgrades will decrease FPS deviation.

I've read that ported piston head will help because it creates a better seal between the piston and cylinder.

Also read that bearing piston head helps because it reduces spring wrap on the piston head.
If this is true, why does that help reduce deviation.

Are those assesments accurate?

What else will help, and why?
bearing piston guide?
different cylinder?
bore up cylinder?
diferent grease?
Air Nozzle?
Cylinder head?

[mcguyver]

Better BBs is your best bet. You can make every kind of upgrade you want, but if your BBs vary in diameter and weight, then upgrades will pretty much do nothing for you.

Get a BB guaranteed to meet a certain diameter. If this is not possible for you, then try different brands until you find one you like.

Metal Tech and Airsoft Elite BBs are guaranteed to be 5.98mm.

[BBS]

good hop-up/rubber, clean inner barrel...also try adding a tightbore barrel to see if it works. yeah and high quality BBs works magic. and don't go full auto, shooting semi with allow 1 second between each shot will increase the fps consistency too.

[ThunderCactus]

better BBs, a tightbore barrel, SPRING GUIDE AND BEARING PISTON HEAD
theres a long explanation, suffice it to say bearings on your spring make your gun ALOT more consistent (my UMP went from 160-220 to a consistent 280 with piston/cylinder head, and a bearing spring guide)