Go to a hardware store and buy some of the expander plugs that is used in holes drilled in concrete to put lag bolts in, they have all differant sizes, as well as ones used to hang pictures on walls, put them in from the rear and tighten but don't over tighten, then tap it out with a rod from the muzzle.you can even fit them by cutting them down on a lathe for differant size cartridges, a custom set.
