Flygonial shared this on Thingiverse:

It is compatible with 130 to 180 sized DC brushed motors (possibly only 130 as a pusher though it may fit without a voltmeter), and accepts most magazines compatible with Hasbro Nerf blasters (compatibility with Buzz Bee mags is not guaranteed). Additional design features include a horizontal flywheel cage, 14mm bore, flat-top with a full-length of picatinny rail, and top-loadability. It accepts full sized or submini microswitches for its flywheel and pusher switches, but only submini sized ones for its cycle control switch.