One thing to add is that what people think is "excessive head space" on these rifles is really built into them on purpose. The 303 Brit is a rimmed case and head spaces on the rim and not the shoulder of the case. The chambers of these rifles are pretty sloppy on purpose so that they will work in the mud, blood and slop of the battle field. Now when it comes to reloading for them the best thing to do is fire form the case and then just neck size them. You will get much better accuracy and the case life if MUCH longer. If you full length size these cases lots of times you will only get maybe 2 or 3 loadings before you get case head separation. IMR 4064, RL 15 and Varget works well with the 174/180 gr bullets.
One other little bit of history is that these were one of the easiest actions to shoot really fast and accurate. 10 rounds in the magazine and the shorter slicker operating bolt a squad of men could really lay down a field of fire then quickly reload with two 5 round stripper clips and go at it again.