Have an older REM Mod 722 in 222 Rem.  The action is long for that cartridge and has a spacer ahead of the follower.
The spacer limits the COL that is possible.  Most all bullets shoot WAY  better when seated longer than will work in the mag.
I would suppose that a 721 in 222 Mag doesn't have the shorter follower and spacer.
Any suggestions.
 
I'll try some things but if things are normal my cobble jobs won't work out.
Maybe, as I think about it, a follower from a M700 223 may well work.
Thanks for listening