Hornady Lock-N-Load Oal Gauges Lnl Oal Gauge CurvedProduct LineThe Lock-N-Load® O.A.L. Gauge is universally acclaimed as the most accurate, easy-to-use and reliable method of regulating seating depth and the resulting “jump” to the rifling.Straight ModelItem #C1000For all bolt-action and single-shot firearms, or any firearm with straight-line access to the chamber. This model is preferred, as it is most precise and user friendly.Curved ModelItem #C1550For all autoloader, lever-action, or pump-action rifles. Also fits all bolt-action and single-shot firearms. Inserts into chamber through ejection port. Not compatible with Adapter “D” or short-bodied cases.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.