Barrel "slugging"

[BoydAllen]

A friend makes a practice on "mapping" barrels using cast in the muzzle laps. He has gotten pretty fast at doing this, and it has ended up saving him a lot of trouble both when diagnosing customer barrels and evaluating blanks prior to rebarreling. When working with long bullets at high pressures getting a good match between interior dimensions and bullets can be important, as is lapped finish. He has also done a considerable amount remedial lapping, something that I would not recommend to those that are not at the top as far as mechanical ability, or anyone who is not to prepared for a long, demanding learning process. He is a former race engine builder with very good problem solving ability and is very good with his hands. The measuring part is pretty straightforward, the lapping part anything but. He uses a slim carbon fiber rod with a flat ended brass jag and light tapping strokes to expand the end of a pure lead lap. He has found problems with a couple of the very best barrels, one after fighting it for quite a few rounds. It had a loose spot a few inches from the muzzle. The manufacturer replaced it without argument. You can feel things slugging that you cannot easily detect by any other method. (The barrel manufacturer that I discussed this with agreed.) Experience with loose spots at the ends that you can measure can be helpful for estimating loose spots that are down the bore, by comparing how much effort it takes to push a tapped up slug into an adjacent tighter area.