I don't care for PPU brass for fireforming in my K-Hornet (a Winchester 54). That gun has a very tight chamber although the camming action of the bolt does get them in ok. When the stash of PPU's is used up I'll try to avoid them in future for that purpose. Others of my Hornets don't seem to care, but their chambers are looser.

"You can lead a man to logic, but you cannot make him think." Joe Harz
"Always certain, often right." Keith McCafferty