The Hornady Lock-N-Load Bullet Comparator is perfect for the reloader looking to quickly and precisely measure seating depth of loaded ammunition from the bullet ogive to the base of a loaded round to eliminate variation in seating depth. The comparator attaches to any caliper and measures cartridges from 17 to 45 caliber. When used in conjunction with the Hornady Lock-N-Load Overall Length Gage (Sold Separately) you are able to determine the exact bullet seating depth for the ultimate precision.