Two things:
First off, you will get that effect if the .303 round was fired in a .30-40 Krag. And, believe me, you get some wonderfully strange things happening when you cram a .303 into a Krag and let fly. Had to remove the front of a Krag casing from the chamber of a '95 Winchester a few years ago.
Second, yeah, there were some pretty big chambers made, but such was not the original intent. I ran into one, 'way back when, got around it by resizing my brass in 2 stages, first in the SEATING die, then in the full-length die. Worked.