Primer set back can occur because there is no powder charge to slam the primer back into the breechface. And the primer charge pressure can move the primer back a little unless the flash hole is enlarged a bit (think small drill bit run thru each case) to prevent the primer charge pressure from "setting back " the primer. In a revolver, this setback, if it occurs could "tie up" a revolver and prevent cylinder rotation. Be sure to mark these few cases you prepare this way. A lot of this depends on how tightly the primers seat into the primer seats. I used rubber bullets in a .45 acp autoloader and of course I didn't care about primer setback. But in a revolver, this may need to be done. Mark the case heads with a sharpie and don't mix these cases up with your regular cases with normal primer flashholes. Of course we are assuming you are a reloader to start with. Some may think these bullets are loaded ammunition which of course they are not. They are just rubber bullets to load into cases by reloaders. Of course the rubber buckshot loads I pointed to are complete loaded shells ready to go.