What hand lapping actually does to a barrel
Lapping is the last step inside the bore. It evens out the diameters from breech to muzzle and leaves a smooth finish that runs the same way the bullet travels.
How it's done
Lilja's FAQ describes the process: a lap is cast around a rod inside the barrel so it takes the exact shape of that barrel's rifling. The lap is charged with abrasive compound and oiled, then pushed and pulled through the full length of the bore, again and again. It gets recharged often, and several new laps may be cast before the barrel is done. The person lapping judges progress by feel, by borescope and by measuring the land and groove diameters.
Why I lapped every barrel
From my time making barrels: the whole point of hand lapping is to get rid of the reamer marks. It basically polishes the inside of the barrel. If you leave those reamer scratches in there, carbon builds up in the little cavities they leave. Over time that can add more wear to the barrel, and it means you end up cleaning it more often.
What it fixes
- Tight spots and uneven diameter. Lapping brings the bore up to final size and evens it out along the length (Lilja).
- Tool marks. Lilja says its process removes tool marks and leaves a finish running parallel with the rifling.
- Fouling. Makers report that well-lapped bores pick up less copper fouling (Lilja).
Bartlein's FAQ adds that its barrels are lapped or honed before rifling and finish-lapped after, including gain-twist barrels.
Should you fire-lap a new custom barrel?
The custom makers quoted here say no. Bartlein's FAQ says fire lapping won't make its barrels any smoother, and Lilja discourages any later internal finishing on its barrels. Lilja also says internal tampering can void its warranty.
Sources
This is general information, not gunsmithing advice. Have a qualified gunsmith fit and headspace any barrel.