Short answer, you don't. It will move after a few shots. A couple of times I have fired one barrel and then when I lowered the gun had the shot from the unfired charge roll out. However, the way in which you reload a double ML shotgun helps preclude it. While you're reloading the R barrel the RR goes down the L barrel between uses. While you do this you're retamping the unfired charge. So the unfired charge will be reset in the bore during reloading. This is how I, and pretty much everyone I've talked to does it with doubles.