bump stock(A "bump stock" replaces a rifle's standard stock, which is the part held against the shoulder. It frees the weapon to slide back and forth rapidly, harnessing the energy from the kickback shooters feel when the weapon fires. Miyer)
collapsible stock(A collapsible stock by definition is one that is not fixed permanently in length, and it does not fold. The collapsing movement is achieved as the stock extends or retracts in a straight line from the weapon.solitaire)
