Don't need to shoot or change anything to the rimmed ctg. The #3 & #4 extractors will work with anything from .45/70's to .22 Hornets, thus 7x57's rim is just fine. Wonderful design, the Enfield.

You will want to keep loads to about 50,000psi, thou - not 57,000PSI as standard CIP for the round. The #3 will certainly handle original factory level 2,350fps loads with 175 RN's.

The magazine is the only trouble - sometimes.

I've a number of friends who converted #4's to .223's, without anything but a small extractor cut in the back end of the barrel. They headspaced off the shoulder as normal for rimless rounds. The empties would just drop into an empty magazine box and that would hold a bunch of .223's before needing emptying, when gopher shooting.

With the 7x57,should be easy to get the factory magazine to work.