seek wisdom on a no4 enfeild

[CINDERS]

Forget the file thingy that is to much bubba with those forces, get new brass though a couple of things spring to mind may be they need to be shoulder bumped as the shoulder may have moved forward on some cases you do not say if you FLS the brass, your friends brass could be differently fire formed but I think the specs should let them chamber in your rifle but maybe not......

The only other thing I can really hazard with a long shot on is a carbon ring build up just out of the case neck where the projie and the case separate there may be some blow by before full obturation at the neck/shoulder is achieved.
This can happen with the smaller cases like the .223's and is an easy fix with a good solvent, 7.7mm nylon brush and some Iosso paste then scrub just that area first, clean it out and do it to the bore and neck area it is very good at removing Carbon and wont hurt the bore.

One book I have states(Nick Harvey- 9th Ed) 303 British Max case length = 56.43mm (2.2215") Trim to Length 56.18mm(2.210") anyway Patricks advice is pretty spot on if you are using hot loads then of course the brass will die very quickly.

Last bit it get it to a gunsmith and bore scope the chamber stranger things have happened.