Any need at all to chamfer straight wall brass when you you are going to flare the mouth anyway with an M die?

Nope.

Don

If the brass has been trimmed a light chamfer to remove the sharp edge, not trimmed the M die is all you need.

Note: Nobody trims handgun brass, so no need to chamfer.

Don

I'm a little anal. I do slightly chamfer NEW handgun brass before the first loading; after that no trimming is necessary therefore no more chamfering. But I only slightly chamfer to smooth up the edges.