[Unclenick]

Re-reading the OP, I think you are right. He's bulging during seating. Probably bullet tilt, as you suggest. The other possibility is slightly longer cases getting over-crimped.

Usually, when I see someone buying a separate Lyman M-die it is for bottleneck rifle cartridge loading to help eliminate finished cartridge runout. The Lyman bottleneck rifle die sets don't have an M-die included. That said, at one point I got one of the Lyman Multi-expander dies, which is a powder-through die with interchangeable expanders for every common handgun bullet from .32 to .45. I've found it useful for a couple of situations where I had inherited dies or some other variation on that theme.

The UniqueTek company sells Dillon powder measure drop/operating tubes with the M-profile for pistol calibers, and that will help people having the bulging problem in Dillon presses.