I'm not familiar with this mod, but the BATF has a standard they (in theory, and most of the time) use for what they allow manufacturers to sell to civilians: a trained machinist with a normal machine shop has to take more than N hours to do the conversion. That's why a whole lot of open bolt derived from sub-machinegun designs had to be changed to fire from a closed bolt with a separate firing pin (the normal submachine gun design as I understand it, at least as of WWII state of the art, has a fixed firing pin in the face of the bolt, the H&K MP5 being a notable post-WWII exception).