A few years ago just for fun I cobbled together some KAC MasterKey styled configurations. Noveske N4 SBR and 16" HK MR556A1 hosting a 7" Rem 870 3" magnum. The Mr. Obvious problem with these configurations are there are not sighting systems for the shotgun, ~ 4 inch offset from the carbine barrel, point shooting a 12 gauge load in close quarters when the shot collar has not even released yet!