To use the Lock-N-Load O.A.L. Gauge, you will need a Modified Case (sold separately) to fit your firearm's chamber and thread it onto the gauge. These specially prepared modified cases are threaded at the case-head and have a .002" oversized neck to accept the same bullets you intend to load. This is critical, as the Lock-N-Load O.A.L. Gauge dimensions obtained will be accurate to .001". These results cannot be attained with competitive products, which use a simulated bullet with an arbitrary shape. Lock-N-Load Modified Cases are readily available to fit most popular rifle chambers. All are manufactured to SAAMI specs.