Many years ago I loaded a box of 20 243 cartridges. ... Still don't know what caused the severely blown primer, but the case has sat on my reloading bench for many years as a reminder.
Bottom line, the previous seventeen rounds everything appeared to be normal and then it wasn't...
You may have witnessed the pressure excursion that the 243 is infamous for, especially if the throat was worn. My theory is it is due to the powders used, at that time some type of bulk powder similar to 7828 and to throat wear and variations. The same thought on the 7RM which had the pressure reduced after it exhibited greater than acceptable pressure excursions. I have loaded many hundred of these two rounds and not witnessed this phenomena. The 243 was plenty worn and the 7 RM I shot the barrel out at around 3,000-3,500 rounds. It was probably gone before but this was at the point it started to shoot erratically and larger groups across the board.
I believe what Charlie Sisk was saying is that a tighter chamber doesn't show pressure signs or blow primers as soon as a looser chamber would. Is the Ruger Number 1 considered to handle pressure well and not show indications as much? I was getting flat primers with a #1 thought it was pressure but turned out to be tight primer pockets on new Nosler brass, the primers were getting the final seating when chambered. When shot they backed out some and were flatter than they should be. But an unfired round looked flatter too after clambering.
