Just to clarify, are you sure you're not confusing the ledge of the chamber throat with crud? If crud is indeed keeping magnum cases from chambering, try a stainless steel brush. These are hard on bores, so bronze brushes should be used there, but sometimes a SS brush will work well in the cylinder. Use a solvent like Shooter's Choice. The belled case trick discussed above is effective as well.
Last edited by Goblin; 04-19-2017 at 04:49 PM.