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 the 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. Fits: Any set of calipers Function: Designed to be used the Hornady Lock-N-Load Bullet Comparator Inserts (Sold Separately) and the Hornady Lock-N-Load Overall Length Gage (Sold Separately) Material: Aluminum and Brass Notes: This is the comparator body only. Bullet inserts sold separately. Slot on comparator body is cut off-center To position the comparator with the Hornady Lock-N-Load Overall Length Gage, install comparator body to the caliper with the screw through the thicker leg of the body. This allows the proper off center alignment of the Length Gage.