When Sister Boniface is inspecting the bullet, she says "9mm, to be my guess". Given the period, it would be much more likely to be .303 British (firing a bullet about 7.9 mm in diameter) or similar. 9mm was (and is) generally a pistol caliber, and while some carbines would fire 9mm bullets, they would use pistol cartridges, with bullets that would be much shorter and less streamlined than the rifle bullet Sister Boniface is looking at.