The Lock N Load headspace gauge is the best method we've found to extend brass life, improve accuracy and enhance safety. The gauge measures variations in your brass before and after firing or resizing. It allows for comparison between fire-formed brass from your gun and your re-sized brass. Any of five interchangeable bushing are attached to your caliper using the same body as found on our Comparator. This kit comes with five bushings that allow the user to check most bottleneck cases, from .17Remington through the belted magnums. Ackley improved chambers use the same bushing as the parent case.