Be sure to replace the firing pin spring in a Rem. 721/722. These guns have not been built since 1964, 56 years the spring is not dependable. A 700 spring will work, I have a SA 700 spring in a 722 that works good.
When I inherited the 722 Rem. .222 Rem. from my dad 1994, He said my loads were to hot and smoke and flame was coming back around the bolt sometimes. The firing pin spring was so weak the firing pin was being forced back into the bolt and the primers were perforated, about 1 in 5 shells. The rifle shot 3 inch 100yd groups. With a new spring the rifle went back to 3/4" at 100yd.