I sized my 41 Mag bullets at .411" and all went well. My castings weren't Keith-type bullets with long front drive bands, though. In all three of my 41s (2 BHs and a 657) all throats passed a .410" minus pin gauge and would not pass a .411", IIRC. I concur that the makers turn 41s out correctly IME. (Another 657 wasn't gauged, I was talked out of it in short order. It shot great.)What you will find with "41's" is far more consistency in chambers and dimensions. At least in the 6/7 guns I have had this has been the case.
.410 has been very good and fits my BlkHawk throats nicely.
CW