an opinion, boron is extremely hard. If you could get it to adhere to a bullet, 1. would it be in a natural state, 2. ionic state, 3 blasted into the bullet case. I have worked with Boron for several years. It is difficult to work with. My experience has been to get it in the ionic form with a electrically charged carrier and dip it in the solution. Then it is in the smallest size possible with no other chemicals to get in the way.
thinking another way, hard bullet vs. soft barrel who wins. or hard bullet vs hard barrel surface = velocity and less wear, possibly greater accuracy due to lack of vibration harmonics in the barrel.