.. the fact it has a shoulder means you can size them as if they were rimless bottle neck cases . I'd think neck only sizing would extend life considerably which I guess double is but would think 4 or 5 times that would be possible .
That it, essentially what I do with neck sizing. I don't try to get a "perfect" fit to the shoulder, as I have a couple .303s and they're slightly different. I don't shoot .303 that much, so I don't have any brass I know I've loaded more than 4-6 times, might get more loadings if I was more precise, I don't know.
One thing about the .303, even "once fired" brass could be on its last legs. Usually isn't, but "usually" isn't "always".
One bag I got had a case head let go on its first firing (by me), and that's when I made up my mind about two things. "Once fired brass" gets FL sized, once, and if it survives the next firing, gets neck sized after that until the fail (or I see they are about to fail) usually its partial case head separation.
The other thing I decided was that when I felt I needed more brass, to spend the $ and buy NEW brass. And when I do, load that with all the tricks I know to maximize case life.
Military rifles (and machineguns) are the worst platforms for case life. Some, like many Mausers are fair to good, like sporting rifles, but many are not. The description one usually reads is "generous" chambers so they will function in dirty combat conditions. Good for the military, bad for reloaders, but then, the military only cares about a case going into the chamber (and coming out) ONCE.