A muzzle attachment for a shotgun is disclosed including a first portion that has a first end, a second end, a central axis extending from the first end to the second end, and a through bore extending from the first end to the second end, concentric about the axis. The plates are angled upward and rearward, with adjacent ones of the plates having forward sections of one overlapping rearward sections of the other. The chest-positionerpad unit provides greater control of compression positioning providing adequate and reproducible chest compressions while preventing rib fractures and damage to vital internal organs. The motion refinement module can operate in a plurality of selected modes including a first mode corresponding to a first sub-pixel resolution and a second mode corresponding to a second sub-pixel resolution.