the CM16 SHOULD have the exact same gearbox shell as your scar, if the CA scar is a clone of the g&g. It will have the holes drilled in it for the selector switches.
If you were to buy a cm16 and learn to tech on it and built a mutant gun, it's a simple matter just to transfer the gearbox into the scar and have a kickass scar. Meanwhile put the scar's gearbox into the cm16.
The only shitty bit is the wiring harness is special on the scar and the selector plate, which on v2s can be a bit delicate to remove without taking the gearbox apart. The selector plate on the g&g scar is proprietary, break it and it's a special order part.
Also keep track of all the screws you pull out... and be careful of tiny springs and ball bearings. I always get a 'I fixed it myself' gun that's missing shit.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.