To put a Marlin XS7 barrel, 223, on a Savage M10 action, I ground ~.2" off the FRONT of a Savage barrel nut. Everything fit, headspaced fine.

This after hours of assembling, measuring. disassembling, measuring... The Savage barrel nut front end goes over the unthreaded part of the Savage barrel---it does NOT fit over the too-big unthreaded part of the Marlin barrel.

NOTE: A FL sized 223 case protrudes from the/a Savage barrel ~.130"/.140"; and from a Marlin barrel ~.150"-.160" .