WHAT DOES BCM LOWER RECEIVER MEAN?

bcn complete lowerExcellent product. Only blem is a small scratch on the best from the qd conclusion sling mount from the castle nut. No major offer to me and saved $90. Much less expensive than developing my spikes tactical lower with bcm sections. Furthermore you have a stacked castle nut and the qd sling attachment. Glad PA acquired these in sto