Getting back into airsoft :) Got given a gun that i need help with.

[flatline]

Not played for four or five years but my mate moved away and gave me a gun he was building and a TM pistol, i need help deciding on parts for it. Its an M4 with v2 gearbox.

I've seen a full V2 gearbox by tomtac that has quick release mechanism for the spring which would be good as i will be playing on a private site also,are they any good? Ive also got a guarder 120 tune up kit and an empty gearbox.

I bought a tightbore barrel and a prometheus perfect metal hop up chamber but the hole for the screw isn't threaded and its also longer than normal, does this use something other than a screw? Also it came with a small o-ring i have no idea what its for.

All the mags that came with it are 30 round so i will need to buy some 300 round mags, need some good quality ones that wont break the bank balance.

Also need a charger and batteries, batteries will have to be skinny ones.

Cheers in advance