Seems like most people on this thread have had good experiences with hBN unfortunately I have not. Quite the opposite. My experience was overall terrible and I would strongly recommend against people using hBN. I actually used this thread and several others as a guide on how to properly apply it to my barrel and coated 500 rounds with it. So, it's not like I didn't give it a fair try.
Initially, what I noticed was that bullets were a little harder to insert and my bullet seating die would just ever so slightly dent the tip of my bullets. A perfect little concentric ring around the tip. Didn't happen to all of them but I definitely didn't like it. Not sure if from an accuracy stand point it actually made a difference or not. I shot them anyway.
The next thing I noticed is that they are HARD to pull. Like ridiculously unacceptably hard to pull. In fact, the force required to pull them destroys the bullet. The collet literally can't get a tight enough grip to pull the the bullet unless you cinch it down to the point that it severely deforms/dents the jacket. The reason I think this is 2 fold. 1) hBN is crystalline. As such it gets embedded in the jacket and interposed between the jacket and the brass. Well, there's not much extra space for it so it seems like it creates an absurd amount of neck tension. Combine this with the fact that it librucates the bullet jacket and you get a situation where you're trying to pull a lubed up bullet out of a case that it is literally wedged into. Suffice it to say they don't smoothly come out. Not only are the jackets deformed but you can see linear streaks where the jacket was scratched by the crystalline hBN. When this first happened I was like "Well, if the hype is true, I guess I'll just shoot them out."
So how'd they shoot? Initially, they shot essentially exactly the same as if I was shooting naked bullets except that every now and then I'd get a ridiculous flier. Like it'd be off by 3-4 MOA. I thought maybe I just wasn't being retentive enough with my reloading practices but realistically speaking that's not true. I'm pretty retentive and this gun shoots 1/4 MOA all day. Maybe better. I mean it's a 243 Winchester that literally puts 3 bullets nearly in the same hole. So if you measure the outside diameter at .30 and subtract .243 then it'd be .06. Technically speaking that's better than .1 MOA. It's an accurate rifle. It doesn't really seem to shoot fliers and I feel fortunate to own it. It's a flippin hammer.
What I noticed is that a couple days/ week later when I went and stretched her out a bit was that my relative group size had started to expand. Initially at 350 I was shooting 3/4 MOA and at 475 I was shooting 3/4-1 MOA. Given the location and shooting conditions I was shooting in I chalked it up to the environment and was initially still happy. So I went home and loaded 200 rounds for the next time I went out to shoot. Then came the severe sad face.
A month later (yesterday) I went back and started shooting at 475. I hadn't touched the rifle since and expected it to deliver similar results. No such luck. My groups had opened up to 6 MOA. I shot several more. Still the same. I dropped back to 100 yards. Still the same. I was like ***. I thought maybe I just needed to clean it since I'd put ~ 200 rounds down it but deep down I didn't actually think that was the likely culprit given the fact that it happened so suddenly. Regardless I packed up, went home, and cleaned the **** out of it to give the hBN the benefit of the doubt.
I went back to the range that afternoon and shot five 5 shot groups and they were all anywhere from 4-6 MOA. Definitely wasn't the rifle needing to be cleaned. I packed up, went home, and pulled 20 rounds. Rechecked my powder charge, made sure my primers were all seated completely and reloaded them (the same cases that I literally just took an hBN coated bullet out of) with naked bullets. I then prepped and loaded 6 more cases from scratch as a control.
I went back to the range this morning and first group I shot measured ~1/4 MOA. Cold bore. The next was 3 pretty much in the same hole. Shot a group with the control lot and again the same. The rifle is a hummer. The problem was/is the hBN. What I think happened is that since it's a crystalline material over ~ a month it literally fused with itself in the case neck and since it was wedged in there my neck tension was both ridiculously high and variable depending on how much hBN was interposed and how much had fused together. This caused all sorts of pressure variations and instead of my ES and SD coming down to single digits they went through the roof. Good to know, but a painful and somewhat expensive lesson to learn.
Time to pull a bunch of bullets!
