I got to admit its very interesting how ever it puts me in the mind set of a P90 with the Mag on top as pictured and th caliber 5.7 reminds me of the FN 57 pistol which is of course 5.7.
It uses the p90 mags. Thought it was a slick set up. Dumps spent brass through the magazine well.
Not my idea of a "Puch Punch your out" setup but it would be nice to show off at the range or to friends. Ya should Check out the .50 Beowulf uppers for AR-15 (M4) system. That packs a punch dude.
I saw those. Bad ass they also make a 50BMG upper.