The Hornady Lock-N-Load Overall Length Gauge is one of the best kept secrets to accurate reloading. This tool allows reloaders to quickly and easily determine the "proper" bullet seating depth in their firearm by accurately measuring their rifles freebore. By having an accurate measurement of freebore reloaders can significantly improve their rifles accuracy by tailoring their bullet seating depth to their rifle. A few thousandths of an inch change between the bullet and the rifling can make the difference between just so-so accuracy and that tack-driver performance that leads to bug-hole groups. To use the Lock-N-Load Overall Length Gauge Hornady caliber-specific Overall Length Gauge Modified Cases are required for use (sold separately). These Overall Length Gauges Modified Cases are made for all common cases from the 17 Remington to the 500 Nitro Express and are oversized to accept the same bullets your reloading.