Hey folks,
So I've been building (and still building) a Masterkey and figured I would post what I've got so far.
As for the details, this is what I've got:
The guns:
1. GHK M4 with 20" G&P barrel, MOE rifle length hand guard, SAI bolt carrier, VFC QRS stock, G&P repro ELCAN scope, Real Steel Grade AR-15 M4 M16 Tactical Tri-Rail Barrel Mount.
2. CYMA M870 tri-shot shotgun with Laylax full length top rail, Matrix 5rd Shotgun Shell Holder w/ Hook Backing, Matrix Weaver to Weaver adaptor.
What I've found:
I also originally bought a G&P mag tube extension for the shotgun but the threads of both the shotty and the extension are threaded in the same direction so I wasn't able to add it to the gun. Also, the barrel on the CYMA appears to be thicker than other shotguns and because of this, I was not able to get the end bracket to connect to the other component that would fully secure it. However it does appear to work fine as is.
So my advice would be that if you plan to do up a Masterkey, go with something other than the CYMA. The TM M870 would probably be your best bet as that was the original model I was going to go for, but with it having its gas reservoir in the stock, you would need to buy a conversion kit that actually caps off the back of the TMM870 receiver which has been impossible to find. Samoon made them actually but it appears they no longer manufacture the conversion. Your next best thing would be an APS CAM80, although I don't know if they make any shotties that don't eject shells, unless of course you want the shell ejecting experience.
Anyway, this is what I've got so far: