Echo1 ER25K - Best Hop Up Unit???
 

I'm skeptical about purchasing a unit and not really knowing if it will fit. I believe, and this has to be confirmed, I must get a m4/m16 unit? If so I was looking at the prometheus, mad bull and systema m4 units. What should I do?

I have a 6.03 509mm barrel, a kwa 2g bucking + nub. Anyone have suggestions?