I finally got big enough zip ties to go around the M4 and the shotgun and keep them in place. I used 3 large zip ties and 1 smaller one to hold it in place. The shotgun can be pumped without moving back, and stays in place relatively well, There is only a small bit of sliding where the 2 tactical rails touch, but its not a big problem. The zip ties allow me to easily remove the shotgun when I no longer want it attached, as well as re-mount it when I decide to put it back on. The shotgun can be fired with surprising accuracy even though there are no sights for it in the first place, and the fact that its under the barrel. I use my right index finger to fire the M4, and my left index finger to fire the shotgun.