Kevin, base diameter of the Magtech brass is .569; rim is .628; rim thickness is about .062, but not as uniform as the Starline rim. It fits perfectly in the RCBS No. 31 shellholder
I must revise my water capacity measurements. I took the original capacities of both a fired Starline .56-50 case and the Magtech 32-gauge derived case without full-length sizing them; after sizing them in an RCBS full-length sizer die, the capacities are 45.5 grains for the Starline and 53.7 for the Magtech -- an 18 percent gain.
I don't know why it didn't occur to me at the outset to compare sized cases.