The MA37 AR has a 32 round magazine ,its the first in the MA5 models .
the UNSC thought that 32 rounds was suitable for an A.R but latter on relized with the new threat "the covies"
that they needed 60 rounds , this caused jamming to the guns so the magazines were reduced down back to 32 .
Hope this helps.
MA5 OR MA37 32 ROUNDS .
MA5B 60 ROUNDS .
MA5C 32 ROUNDS.
MA5K 30 ROUNDS , THIS IS A CARBINE VARRIANT.