Several technical details would preclude such an option ( using an AC-556 in the place of an M4 ) from the realm of consideration. The two most notable are that 1. this model of rifle requires proprietary magazines and 2. non-standard parts of repair would need to be specifically ordered. ( "...Ruger stopped working on these guns in 2009 and no longer supports them in any way. About 90 percent of the internals swap out for regular Mini-14 parts, but odds are those aren't the ones you broke.") (1)(<https://www.guns.com/news/2014/03/10/ruger-ac556-totally-legal-totally-full-auto-mini-14-video>)