Hey, guys. I used to work on my own guns back in the day, and I know I'm a bit rusty, but I'm having an issue where I'm left scratching my head.
MP5. I wish I could say this were all one brand, but this was one I purchased used years ago (and prompted a bad sellers review). The body is ICS. The gearbox is an older JG Box, gears, etc. I've had to replace the tappet plate, the wiring harness, the trigger, and other misc parts.
I'm currently having an issue where it doesn't matter what spring I put in (110, 120), I cannot seem to exceed 310 fps max. I did upgrade to a Hurricane NB cylinder kit. The cylinder/piston combo had great compression (tested by hand). The only thing that I can visually see is that the cylinder is maybe 1/16" shorter than the space in the gearbox. I've visually inspected the GB and do not see any cracks or gaps elsewhere (especially when firing). My next step is to replace the gearbox, but I wanted to come here first and see if you guys could provide any insight.