Regulating seating depth and the resulting “jump” to the rifling is widely regarded as fundamental to improved accuracy. A few thousandths of an inch change between the bullet and the rifling can make the difference between average accuracy and real tack-driver performance. To use the Lock-N-Load® O.A.L Gauge, you will need a Modified Case (sold separately) to fit your firearms chamber and thread it onto the gauge. Hornady offers interchangeable modified cases in most common sizes from .17 Remington up to and including 500 Nitro. 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