MODIFICATION - APS CAM870 MASTERKEY | AIRSOFT

MODIFICATION - APS CAM870 MASTERKEY | AIRSOFT
I really like to shoot, so I want to put a gun on my gun, so I can shoot while I shoot. Enter: Masterkey! No duct tape allowed.

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STEP 2: DISASSEMBLY
To start off with something simple, I will disassemble the G&P Masterkey kit.
3 screws later, and I’m done.
Next I disassemble the CAM870. For more detailed instructions, click here for my CAM870 disassembly video.
STEP 3: MOCK-UP
I temporarily install the mounting hardware on my M4 by removing the lower handguard, inserting the rear mounting point into the delta ring and closing the quick detach mount around the barrel.
I need to compare the G&P and APS receivers in order to get the mounting points right. If the shotgun is mounted too far forward and it will look odd and be front heavy. Well front heavier, to be correct.
Too far to the rear might damage my M4, and in its current condition it won’t fit.
STEP 4: SAWING
In order for the receiver to fit and not interfere with my M4, I must cut down the stock screw attachment so it is flush with the receivers back end.
STEP 5: MOUNTING HOLE PLACEMENT
The placement of the mounting holes lengthwise is pretty easy since I can use the G&P receiver as a template. I mark out the spots for the new holes using a small implement dipped in white paint.
The front hole is harder to reach, so I use an allen key to get a rough position.
When the paint has dried I put my mounting hardware on the receiver and double check that my markings are on target.
Now I know the position lengthwise. But what about centering?
I could measure the width of the receiver and guesstimate the centre line. But if either the front or rear screws are even slightly off centre, this could radically point the shotgun left, or right. Rendering it useless.
Therefore I measure the width of the CAM870 receiver, and commission a drill guide of the same outer diameter. That way my hole will always be centred, no matter how I position the drill guide.
STEP 6: GUIDE HOLES
I make a 2mm guide hole in the front,
middle and lastly the rear.
I quickly check that the holes line up.
The rear and middle hole is spot on, I say!
But as suspected, the front hole is slightly off. But no worries! I’ll work with the two good holes and adjust the front hole later.
STEP 7: MOUNTING HOLES
With a 4.5 mm drill bit I enlarge the rear guide hole, and then the middle guide hole.
I place the 4.5mm drill bit in the middle hole to keep the mounting hardware in line, while I thread the rear hole through the mounting hardware and receiver in 5mm. Then I insert a long M5 screw. I remove the 4.5mm drill bit and treat the middle hole the same way as the rear hole.
Now, when the middle and rear are affixed, I can see the exact placement for the front hole.
STEP 8: FRONT MOUNTING HOLE
Using the entire mounting hardware as a drill guide, I drill a 4.5mm front hole.
I treat the front hole like the others, thread it in 5mm, and then add an M5 screw.
STEP 9: MAKESHIFT NUT
As a stroke of good luck, the middle hole went clean through the rear screw hole for the blocking plate.
Therefore I will turn the blocking plate into a makeshift nut.
This is easier said than done, since it’s made out of extremely strong material. Going straight for a 4.5mm drill bit was unsuccessful. I needed to gradually use bigger and bigger bits, until I finally got up to 4.5mm.
After this I attach the blocking plate on the inside with the remaining screw, and thread it through the receiver in 5mm.
I confirm that the threading is successful with an M5 screw.
STEP 10: AFFIX PLATE
The shell latch arm retaining plate on the back won't have a stock to hold it in place when the Masterkey is ready.
So I drill a hole through the plate with a 2.5mm bit. This leaves a mark on the receiver, which I also drill.
Next I thread the plate and receiver in 3mm.
A surplus M3 motherboard screw, with wide rim head, will work nicely to keep the plate on the receiver.
STEP 11: JOINING
Time to join the Masterkey mount and the CAM870 receiver.
Since I have enlarge the mounting holes the g&P screws won’t work. So I use two M5x16 socket screws for the rear and middle mounting holes, and a M5x8 countersunk screw for the front. I have cut and modified these screws so they fit and don’t protrude into the bolt slot. At this point I install the front and rear screw. Since the middle screw holds the blocking plate it can not be installed until the bolt is in place.
STEP 12: ASSEMBLY
Now that everything’s is done, I can assemble the Masterkey.

Please note that I’m performing these modifications at my own risk.