I am uncertain what to do with this information. I believe it may have substantial benefit, especially for black powder shooters. So I'll furnish it, and readers can use it as they wish. I am in correspondence with Mike Wright of http://www.hefusa.net/. Mr. Wright has confirmed that HEF's nitriding can be applied to INSIDE DIAMETERS of rifle barrels as long as 36 inches. If wanted, it can also be simultaneously applied to outside surfaces. The exterior finish is a dark gray-black whose ultimate color and texture depends upon finish of metal to which it is applied. This finish can be applied to lock parts also, excepting springs. And heat treat will not be affected so long as you furnish correctly the information they request at time of order.

I do not know whether build-up, if any, is irregular -- to preclude its application to target barrels -- or process-induced warpage/distortion occurs.