The Ballistic Tools case mouth and neck tension gauge tells you exactly how much neck tension your reloads will have. Measuring neck diameter with calipers is an iffy proposition at best. A small burr on the case mouth, a slight angle to the calipers, or even just placing too much tension on the calipers can give distorted readings. Improper neck tension can make projectile insertion more difficult, or even cause a safety issue if the neck is too loose. Using the Ballistic Tools gauge solves all those problems. It can knock off a small burr during insertion, and will not distort the round shape of the mouth like calipers will. This case mouth gauge is a quick and reliable way to test your neck sizing and neck tension.