AR Series Small Base Taper Crimp Die Set (.308 Winchester)

A semi-automatic does not chamber a round the way a bolt gun does. It strips the case from a magazine at speed and slams it into a chamber that was never fitted to your brass, so cases that close easily in a bolt action can leave an AR short of battery. RCBS built this set for that problem. The sizing die resizes cases below SAAMI minimum, reducing the shoulder and body a few thousandths more than a standard full-length die, which RCBS says guarantees the cartridge will rechamber in AR-platform weapons. The seater applies a 7-degree taper crimp, gently squeezing the case mouth straight inward to remove flare without rolling it into the bullet; on a gas gun that crimp is what stops feed-ramp impact and magazine recoil from driving bullets deeper into the case, which raises pressure. RCBS is explicit that this set is not recommended for ammunition used in bolt-action rifles, because the extra sizing works the brass harder and shortens case life for no benefit. Buy it if you actually see failures to go into battery with standard dies, not as a default.