To use Stoney Point's O.A.L. Gauge, you will need to purchase a Modified Case 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 O.A.L. Gauge dimensions obtained will be accurate to .001". Results such as this cannot be attained with competitive products which use a simulated bullet with an arbitrary shape.