by Steve » Tue Apr 28, 2015 2:04 am
There are (or at least have been) conversion kits for maruzen m870's. The thing you are looking for is called a "master key" conversion for a shotgun.
If you can't find a kit at a decent price, I'd suggest removing the pistol grip from whatever shotty you prefer, shortening the barrel if necessary, mounting a chunk of rail to the top of it, and using a rail to rail adapter to hold it to the RIS of your rifle. Or, if you want to get seriously hard-core, pick up a non-RIS M203 and mod the M203 mount to hold the cut-down shotgun so it looks like this: