[lurkingknight]

I'm not a huge fan of CA m4s. The last one that ended up on my workbench had 4 different sized screws holding the grip on in the gearbox. They were really poorly made screws, some had larger threads than others, like the dyes that cut them were newer. What ended up happening and the only reason I found out was that somehow all 4 screw holes at the bottom of the gearbox stripped and none of the screws were able to hold the grip in. The only fix to that is replace the shell or find screws that are slightly larger and rethread the holes.

I was lucky, I had stainless steel screws that had deeper threads than all of the screws that came from factory and they cut new threads into the gearbox.

Aside from that, the gear axle holes are slightly offspec in CA gearboxes, some gearsets have issues fitting without binding.

CA nozzle lengths can sometimes be weird too, causing issues with feeding if you use a nozzle with more proper specced lengths. The hopup unit is cast to fit the nozzle, so if you replace the nozzle the hopup unit doesn't fit right and can cause feeding issues.

Replacing any of those 3 things, gearbox shell, nozzle, hopup unit without knowing which part is offspec vs what you're replacing it with can cause all sorts of headaches, and you may never figure it out either. The only one you can know for sure is nozzle length since you can measure the exact length with calipers. You can't measure the depth of the hopup unit, where the rubber sits in relation to the nozzle or the gearbox shell and how it mounts into the receiver, where it's offspec in relation to this part or that.

This is why I like the g&g combat machine, it has VERY good tolerances to existing aftermarket parts and you can put just about anything into it and it will work. You can even put lonex gearbox shells and parts into it when the shell breaks. I've had aftermarket shells not fit into CA guns before as well.