Another excellent post on the Lee Enfield forum from contributor Parashooter, on headspace basics with the .303. With a lot of old surplus military rifles floating around in who knows what kind of condition, touching base with the basics doesn't hurt. Parashooter makes a good point of not loading your bullets too hot, as it is not infallible, (or a magnum rifle). However the Lee is a robust design nevertheless and even with case failure from excessive headspace there is relatively little drama. He has a neat explanation of what's going on in the rifle's chamber when a round is touched off. With just a little moderation in your loads, you can extend the life of your .303's brass.
Headspace 101 from the Lee Enfield Forum