Dumb idea. It should be a separate weapons system, not slung under an M4.
It can be configured as separate weapon as well.
Hoist the M26 combat shotgun to your shoulder, square your stance, lean toward the target standing before you and blast away. With a yank of the straight-pull bolt action, the weapon is ready for another. Now take a look in action: http://youtu.be/cGDq3c0Hv7c