We usually mill our octagon barrels like one of the two shown below. The most common is with the round cylinder section at the chamber end. When dimensioning octagon barrels we need to know the across-the-flats dimensions at the muzzle and where the radius of the milling cutter starts near the chamber end.
To determine the minimum round diameter required if you know the flats width, multiply the flats dimension by 1.0824. For example a .625″ flat width X 1.0824 = .677″ minimum diameter to clean up milling to a .625″ width. Or if you want to know what a specific diameter will clean up to milling it octagon, multiply the round diameter by .9239.
We do not have standard dimensions for octagon barrels as we do with round barrels. They are all made per the customer’s request. The radius section of an octagon is tighter than our standard round barrels. Sometimes we’re asked to duplicate a standard round barrel but because of this difference in the radius it isn’t practical to do so. The standard numbered round barrels are turned with a 30″ radius. The radius of the milling cutter used to make the octagon barrels is 1.50″. As a result the flat-to-flat dimension just ahead of the radius is usually larger than the corresponding dimension on a round barrel. In the available .pdf drawing below this would be the .875″ dimension.
Unlike our round barrels, the octagon barrels are not finish polished. The finish of the flats is as it comes from the milling cutter. The flats will require some hand polishing.
The above image, made from a solid model, is an example of a typical octagon barrel. To see a .pdf drawing of this model – Click Here
To see a .pdf drawing of this typical double-step octagon – Click Here