My friend Keith has a dedicated DPMS AR15 in .22LR. The magazine is an AR15 magazine-shaped polymer housing that encompasses a small metal magazine. You can see the metal magazine below.

The magazine has a built in ejector and a small tab that sticks out of the back. The tab indexes into the polymer housing to keep the magazine in place.

Does anyone know what magazine this is? What did DPMS use for their A22 rifles? It somewhat resembles a Marlin or Savage 22 magazine but it is not the same. Perhaps this is a custom made magazine just for DPMS?