Have the security of knowing you always have a spare 20-round mag as back-up just in case other ammo sources are not available.US Tactical Systems Back-up Twenty with Push Button Design Have the security of knowing you always have a spare 20-round mag as back-up just in case other ammo sources are not available. The BACK-UP TWENTY™ buttstock adds minimal bulk to the overall weapon, yet the extra magazine may be the difference between life and death. Constructed of durable Dupont® Zytel®, the stock retains the spare magazine using the same latching mechanism as the regular M16A2. The magazine is securely stored in a dust-free environment until the trap door is deliberately opened and the magazine release button is pushed.This stock replaces the standard M16A2 or AR-15 buttstock and maintains the weapons existing handling characteristics. Includes a standard size sling loop. 20 Extra Rounds Durable Dupont Zytel Push Button Design Mounts On M16A1 or AR-15