Project Masterkey

I don't know if this is in the right section or not. If not MODS move it please.
I've decided that instead of doing a M4 stubby, I'm going to do a masterkey instead. I have already picked the shotgun out. The only thing is, gotta figure out how to mount it. Any and all ideas are welcome. I am brainstorming right now trying to think of a way. Thanks to all.

What shotgun?

AGM M500.

Barrel length on those is pretty long. You planning on cutting it down?

I do plan on matching barrel lengths to give it a finished look.

You're only going to be able to get off as much as the feeding tube beneath the barrel goes, and on the AGM its about 3 inches from the end of the barrel. You may want to consider either mounting this under an M16 instead of an M4, or looking for a shorter shotgun.

This could work for an M4, its pricey for a mounted weapon, and it uses shells, but performance wise it would be ideal.
http://shop.ehobbyasia.com/other-gun/shotguns/maruzen-m1100-defender-ii-gas-blow-back-shotgun.html

I saw this vid a while ago. There are videos of other people that have done it too I think.

There have been a few people I have seem do this, and they used the G&P 870 pistol grip shotgun, and modded it to fit. I think G&P ran a short run of actual Masterkey shotguns that could be attached to the underneath of a RIS rail such as a M4, as I saw it for sale on TN airsoft not to long ago. It wouldn't be easy. You may be able to find someone with one of those G&P masterkeys used and buy it but it would be a couple hundred bucks at fair market value.