[Unclenick]

Condor Bravo,

If you buy a box of Lapua cases, you find them factory chamfered. Lapua does this for their commercial loading, too. There are two reasons: one is to help the bullet base self-center over the middle of the neck opening, and the other it to avoid scraping bullets (not an uncommon thing to find on commercial ammo), though they do make the chamfer fairly smooth. As Bart mentioned, you need to burnish the case mouth with something to completely avoid it. I use to run a freshly chamfered case over a carbide expander a couple of times to accomplish that, but Bart's E-Z Out method is better and I've been using it since he first posted the idea.

The M-die expander is a little bit of a different animal. The small step it forms near the mouth of the neck helps bullets start into the neck straighter so the finished cartridge tends to have less runout on a concentricity gauge, provided you were careful not to bend the necks in the first place. It can allow a standard seating die to perform about as well as one of the benchrest/competition type seating dies do with just chamfered cases. The downside is more neck working. You can expect to anneal more often to prevent neck splits that start at the case mouth. You also, especially for a self-loader, want to set the crimp shoulder in the seating die down enough to iron the step out, which can be ignored with standard loading of a chamfered case, and which also adds to more working of the case mouth brass.