I am not 100%, but I don't think a 223/204 mag can possibly be the same as a 308 family mag. You will want a 308 family mag. When I had my 450 on a DBM rifle action it was a 308 mag. It fed the first two round great, but had issues with the third round. Search for bootsmcguire posts about his 450 build he had his running great. My 450 is on a striker and set up as a single shot now.