dr. mabuse,
I've dealt with Boronitride a little bit with semiconductor research and development. I don't know how easy it would be to do on the barrel of a firearm as all the surfaces I see it on are flat silicon surfaces.
There are different crystalline forms of BN and you probably want cubic lattice form of BN as opposed to the hexagonal form or the amorphous form. Hexagonal form is more like graphite while cubic form is more like diamond due to the crystalline structure. Cubic form is a bit trickier to make than the other forms in my experience.
I know that in industry they use high temperatures and high pressures to coat tools with BN. There are other ways to deposit it such as using lasers, PECVD, sputtering, etc. You might talk to companies that make bits/blades for cutting tools.