gearbox questions
 

Which gearbox is best to build a V2 7mm V3 7mm or V2 8mm V3 8mm?
I'm building it for a M4 and I want Maxium FPS.

V3 will usually outlast V2. That said, some of the newer V2 boxes (like Systema) claim to be able to handle a M170 spring.

Get a systema V2 drop in mechbox.

I like Deep Fire MBs. Systema MBs are really nice too. I have them both. I haven't seen but one MB fail so I still don't see issues with v2 being weak. Its just how far you are going to take the abuse.

sorry but you are limited to a V2 gearbox.
V3 doesnt not fit in a M4 or any AR15 based shell.

Well the bigger your bearings are, the more reliable they will be.
It really comes down to the best brand of mechbox shell and the availability of bearings, Guarder makes a great 6mm bearing mechbox, G&P makes a good 7mm mechbox. As for bearings, well prometheus (best damn bearings I've ever seen in airsoft), makes 6mm and 7mm bearings, but not 8mm as far as i know.

if you want the best, and that can take anything, get a prowin 8mm gearbox, but if you dont want to spend hours making it fit, stay clear.

im just finishing mine up now, 6 logged hours so far.