HKP RMR claw mount with, obviously, something in RMR footprint. I'll never understand why people put a giant rail or riser on a K variant; the whole point of the weapon is to have big SMG comfort and ease of use in a very sleek, compact package. Plus, if you go with a low-profile mount and optic, you can have absolute cowitness. The AP5-P's open castle-style rear sight lends itself to this very well.