The Allen Engineering AEM5 is the direct fit replacement for your MK12 SPR. The AEM5 attaches to the weapon via a mount (not included) and a collar that goes over the barrel for alignment. The suppressor mounts via threads on the outside of the mount centering on the adapter fitting. A 20 degree taper on the adapter locks the silencer onto the barrel and seals gasses inside the suppressor along with preventing the suppressor from coming loose under use. While the AEM5 is quite long at 8.8″, it only adds 4.5″ past the end of the muzzle device.