I know the difference between rimless, rimmed, and rimfire but I thought the .303 was a rimfire.
I'm too cheap to buy match grade stuff anyway but when I start reloading maybe I'll fiddle with making some.
The only difference in .303 is that it might be Bredan primed instead of Boxer primed. The ammo store that I frequent has some metric designated .303 British. I forgot what the numbers are and I can't find the piece of brass that started my search as to who sold the stuff in my area.