Ive been using warhead industries 0.32 and they work great in my MTW, can also buy bulk from them which works out to like £12 a kg which is very good for 0.32
For any GBBR you need to be using really good, hard ammo. Geoffs is the best or G&G are pretty good. AEGs will take any old muck but GBBR are more picky.