The Small Base Sizer Die is for bottleneck type cases used in tight-chambering semi-automatic, pump or lever-action firearms. The sizer resizes cases below SAAMI minimums, reducing the shoulder and body by a few thousandths. It also de-primes the case. Note: Not recommended for ammunition used in bolt-action rifles. Sizes brass smaller than SAAMI specs Bottleneck type cases semi-automatic like AR-15 Reduces shoulder and body by a few thousandths De-primes the case The RCBS Small Base Sizing Die is special in that it sizes the brass smaller than SAAMI specifications. This helps aid greater reliability in semi-automatic rifles like the AR-15 pattern rifle and more. If feeding reliability is a must in your semi-auto or if you're having chambering issues with your semi-auto - this is the sizing die for you. Component and Caliber Reference Table