I fireform 375 brass to 416 Rem Mag like this:
Run the case into the 458 Lott fl sizer die with the decap and expander ball removed.
Prime
Put about 10 gr of bullseye or red dot or some other fast piston/shotgun powder into the case.
Fill the case with cream of wheat.
Take a bar of soap and press it down over the mouth of the case so about 1/2" of soap forms a plug in the mouth of the case, and compresses the cream of wheat firmly on top of the powder.
Single load the case, point it straight up and fire. (outside at the range)
You should take out a fire formed case that you can now trim to length and run through the full length sizer die.
