.303 Enfields are notorious for having rimlock. I lost first place in a WWII shoot at Fort Meade years ago when my No4 rifle locked up. I had to dump the magazine and reload dropping me into second place. The way to avoid rimlock is to load your stripper clips properly and always load from a stripper clip. It has been over 25 years since I competed with Enfields so I don't remember how to stagger the rounds in the stripper clip properly. I'm willing to bet someone has put it on the internet somewhere.