The boolits I've done this method with are RCBS 25-100 & a clone of Lyman's 311041 and have seen no signs of slumping. However the resulting bullets are "harder than the hubs of hell". The alloy used is tested Saeco 5 before powder coating and heat treating and ended up as Saeco 11!
My concern is that with the 25-100's, when I went to size them from .262 to .258, some of the coating was rubbed off, exposing a lead streak lengthwise along 2 sides of the bullet. The lead streaks were on both sides of the bullet and apparently 180 degrees from each other. Is this an indication that the Lyman H die is out of round?
I have not played with the 041 clone yet.
If I size the 25-100 to .258 first, apply a gas check, and then powder coat, the end result is a bullet that mikes .258. I only wish I hadn't powder coated so many before sizing them!