As always... Para comes through with the good stuff.  Many thanks Para!

This all boils down to misinformation, misunderstanding, and downright confusion.  As FredC says, there are too many unknowns.  Was it REALLY chambered for .303, or did some unsuspecting or uneducated shooter simply not know the difference (to the untrained eye, at first glance, one could easily mistake .303 for 30-40).  Without actually speaking to the first guy sticking a .303 into it... and without actually measuring the chamber/barrel/receiver/etc., there are too many things left to chance!