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The Art Of Barrel Reboring

Barrel reboring is one of those obscure but vital services to the gun trade that gives life to all manner of restoration, repair and custom gunmaking projects. This short column will hardly do the subject justice but it is important just to touch on it. Reboring, as opposed to making a rifled tube from scratch, is simply removing existing rifling and cutting new rifling in a larger caliber. Believe me, use of the word “simply” is highly misleading because the process is a complicated art that takes a lifetime to master. Which is why, important as it is, there are few successful practitioners.

The reboring process is simple in theory but equipment-rich and heavily nuanced, requiring a sophisticated understanding of machine-tool processes. A barrel is fixtured on a lathe-like machine. Then, a piloted counter-bore (a guided, square-ended drill) is run into the barrel to remove the bulk of the material between the existing bore and the new rifling.

To smooth this cut and bring it to final size, an unpiloted reamer is run through next. At this point, reborers may hand lap the barrel, a precision polishing process to smooth out imperfections and tool marks and to ascertain a consistent bore diameter all the way through—critical to good accuracy in a barrel.

Last, the tube is rifled with a special cutting head with an adjustable cutter. Multiple small, progressively deeper cuts are made until the target groove depth is achieved. The last step is to lap the finished product. The process may involve two or three precision machines with high-pressure tool lubrication systems and countless bits of specialized tooling in the form of reamers and cutters, each dedicated to a specific caliber.

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