Use Stub Gauge to Check Shoulder Bump, Monitor Throat Erosion

Forum member Rich DeSimone uses a handy “Stub Gauge” for setting shoulder “bump” and seating depth. The gauge is made from a section of barrel lopped off when the muzzle is crowned. The chambering reamer is run in about 1/4 of the way, enough to capture the neck and shoulder area of the case. Rich then uses his full-length die to “bump” a master case with the ideal amount of headspace for easy feeding and extraction. He takes that case and sets it in this Stub Gauge, and measures from the front of the gauge to the rim. He can then quickly compare any fired case to a his “master” case with optimal headspace. Since the gauge measures off the shoulder datum, this tells him how much to bump his fired brass.

In addition, the Stub Gauge can be used to set bullet seating-depth. Rich has a channel cut transversely on one side of the gauge, exposing the throat area. Since the interior of the gauge is identical to the chamber in his gun, this lets him see where a seated bullet engages the rifling. He can tinker with bullet seating length until he gets just the right amount of land contact on the bullet, confirmed visually. Then he measures the case OAL and sets his seating dies accordingly. This is much handier than using a Hornady Tool to measure distance to the lands.

But what happens when the throat wears and moves out on your live barrel — making the actual length to lands different (slightly longer) than before. Well, the stub gauge is still valuable as a known starting point. As your barrel’s throat wears, you may seat your bullets out further to “chase the lands”, but the gauge provides a constant land engagement point, in the barrel’s “as new” condition. By measuring the difference between the land contact point on the gauge and the actual contact point on your barrel, you can determine throat “migration”.