I HAD THE SAME PROBLEM WITH SOME PMC BRASS IN MY 7MM-08. IT'S NOT THE RIM, IT'S THE NECKS: THEY ARE TOO THICK !!!!!
I got 60rds of PMC factory loads when I bought the rifle new, as I needed some brass for reloading. Even the factory loaded ammo chambered hard due to the excessive neck thickness. Reloaded ammo was even worse, even though I'd trimmed the length to minimum. (The brass was also excessively long, too!)
I would pull down all the loads loaded with the PMC, because to fire it is to risk excessive pressures and rifle damage, particularily with the .243.
I pulled down all mine after I loosened the primer pockets on several cases before I realized what was really going on.
(I presumed some of the other possibilities that others above mentioned, especially that my rifle also has a very short throat).
After neck-turning the brass down to max-spec, it is now my most accurate brass. It took total shaving of 360deg of the neck, for those familiar with neck turning, to remove sufficient brass.
(ie: I removed all the excess brass from the neck, reamed the flash-holes, and uniformed the primer pockets.)
Be careful especially with the .243. Through the years it and the 7mm-RemMag have gotten bad reputations for pressure excursions resulting in busted rifles. I had a friend have this happen with his Rem788 with a previously good performing load. He had "switched" brass however! He split the head of the bolt when a primer blew.
I'm now convinced that it's variation in tolerances in the brass that is causing this.
I have several lot#'s of "same-make" .243 brass that will vary substantially in weight-hence internal capacity. With the high operating pressures and substantial case capacity relative to the bore diameter, the .243 is much more sensitive to such variations in case capacity.
Hence, it is prudent to separate cases by headstamp, and then seperate by weight.
Use great caution with this otherwise excellent cartridge.