NEW SEMI AUTO SLIDE HAMMER RAILS FOR BUILDING A SEMI AUTO ONLY MK46S OR MK48S AND M249 SEMI. ONE PAIR (2) OF RAILS INCLUDED. THESE RAILS ARE THE SLIDE HAMMER GUIDE RAILS AND RESTRICT THE OP-ROD CHOICE TO SEMI AUTO ONLY AND ARE NOT THE BOLT RAILS. PRECISION MACHINING OF CORNERS AND BEVELS, THE BEST QUALITY SLIDE HAMMER RAILS AVAILABLE.