This is a magazine adapter for use with standard G3 magazines. This should fit most HK21 kits with the four holes at the bottom of the feed mech hanger, and the cutout in the side for the magazine catch. You will need a G3 magazine catch kit (an AR15 mag catch spring will work). The magazine catch button will not protrude from the side of the hanger, so you may need to widen the hole in order to get your finger in there; I'm still working on a solution for some sort of extended button.

This may require some fitting to your gun and I cannot guarantee it will fit every single build.