GP M4 help

[Dave1]

Hi I am fixing my friends M4 for him. GP M4 special forces. The gearbox has been reshimmed. As it stripped the beval gear. Then it stripped the piston. Both have been replace with better parts. I have put it back together. Now its developed a weird problem I need your help with. Full auto works fine, safe works fine. But Semi auto is hit or miss. Most of the time it doesn't work. If you go from safe to semi it hardly ever works. If you fire full auto, then goto semi it may fire once and that is it.

Can anybody shed any light on what the problem might be? Any help would be appreciated.

Cheers Dave

[Dave1]

Re: GP M4 help

Hi also the piston sometimes half cocks in semi auto. When semi auto isn't working. Most of the time. It feels like the trigger is locked, just like in safe position.