[Rock Star Sorrow]

Okay so this one might be kind of hard to explain, but basically I'm switching gearboxes and front RIS out of 2 different M4s(1: king arms oberland arms, and 2: the deepfire va custom). Whats going on now is that I've changed everything major so far but when I have the barrel in the gun the top and bottom receiver are off by maybe 2mm causing the hop-up/barrel to not seal properly and the gun to misfire/terrible fps. I've tested the receivers without the barrel in and there's zero problem connecting properly, and like wise testing the hop up and barrel holding it into the gearbox with zero issues firing.

Has anyone else had any issues like this, if so how did you guys fix it.