Sierra seems to have discontinued their moly bullets, as have Berger and Hornady. Norma still uses the original NECO process to make their Black Diamond line. Norma gave the process one of its original boosts when it put photos on its website of two sectioned 6.5×55 barrels that had been fired 10,000 rounds, one with moly-coated bullets and one without. The one fired with bare bullets had essentially no visible rifling for some distance from the freebore into the barrel, while the other still looked pretty good and they had 100-round groups showing it still shot well before they sectioned it. Tests by others in other calibers did not get that same degree of difference consistently, so this seems to be a chambering-by-chambering thing to study.
Moly is insoluble in water, so it doesn't attract moisture. What happened to cause trouble was the original moly-coating process as developed and patented by the late Roger Johnston (NECO), employed laboratory-grade acid-neutralized moly. He could not afford to defend his patent when Wheeler decided to ignore it (Wheeler's owner was an attorney who basically told NECO to go pound sand, as he could afford to keep it in court limbo indefinitely because his legal services were free to himself). Wheeler put out an inferior product with cheap grade moly that had lots of free sulfur in it. Some cheap moly is recovered from metal stamping operations, and that can have free iron in it. Either can combine with moisture to initiate rust. The other budget copycats likewise failed to use acid-neutralized moly.
At that time, people trying moly (I was one, using the more expensive NECO product) found it dramatically reduced metal fouling. I had a Garand barrel that lost accuracy due to heavy copper fouling at about 40 rounds, right in the middle of the 600-yard slow fire phase of the National Match Course. When I switched to moly, the accuracy loss vanished and I could shoot an 80-round match with no accuracy deterioration and there was less time needed to clean it afterward. The fact guns could go so much longer without cleaning fouling caused some to conclude they could even shoot a whole season without cleaning. That was never claimed by NECO, though it might have been possible with their product. Where it was impossible was with people using the cheap junk moly full of sulfur or iron. They got rust pitting trying to get away without post-match cleaning, and did not realize the cause was that they had cheap moly, so they began to badmouth moly in general. That discouraged a lot of people.
That said, hBN really is easier and less dirty. I am unaware of problems with it rusting barrels. I know the powder can start to clump in high humidity. So, like moly, I would still clean after shooting bullets coated with it. I have taken to ignoring the bullets and have been using Tubb Dust, which is hBN you add to powder so it goes down the tube with the powder and you don't have to do any bullet applications. However, it does get everywhere, and coat your powder measure pretty convincingly.
An issue that arises with moly and hBN is that they reduce velocity a little (20-50 fps in high power rifle). Most people think a lubricated bullet should go faster because it lacks friction, but powder depends on resistance by the bullet to provide the degree of confinement it needs to build pressure. This is especially true at the start of the burn, and losing friction means loss of start pressure because the bullet pushes into the throat more easily. The bullet also gets too far forward in the bore too early in the burn, flattening the pressure curve by expanding the space behind it too soon. So you have to increase the original charge weight, usually, half a grain or so, to maintain your original load velocity. And even if you do that, you find out your barrel time has gotten slightly longer because the added powder has slightly raised muzzle pressure so more acceleration occurs late in the bore and less at the pressure peak, which is still below its original value at that point. The bottom line is that charge weights tuned to uncoated bullets don't apply to coated ones. Pretty much, you have to work your loads up over again with coated bullets of either type.
On the plus side, because of that flattening of the pressure curve, the reduced friction lets you get more bullet speed from the same peak pressure. So, when you do retune your load, you are spending a little more per round on powder, but you will end up with higher muzzle velocities at the tuned points.
The next question is, will your retuned loads match the accuracy of your original tuned loads? I haven't been able to tell much difference. I am aware of a test done with Dr. Ken Oehler long ago in which some moly-coated bullets had slightly higher ballistic coefficients (slightly lower TOF at 1000 yards) than their bare counterparts. There was no solid explanation for it, though it was noted that uncoated bullets often have a little tab of copper at the back end of each rifling mark where the copper has been smeared slightly, while recovered coated bullets do not. It was speculated that this may increase bare bullet drag a little. But I don't remember the details of the test, so other factors might be involved. Testing by Sierra in the late '90s and published in Precision Shooting did not indicate any accuracy advantage. Indeed they had a very slight loss of accuracy, but there were no details on whether they adjusted their uncoated bullet load or not. At the time, I suspected they had not retuned it.
So, the bottom line is moly-coating and, I expect, by extension, hBN coating has some pluses in terms of cleaning ease and eliminating fouling shots for settling the gun down. Accuracy improvements were found by some, but not by others and I have no way to distinguish the cause being the bullet or failure to retune the load for the different barrel time. Some chamberings get extended barrel life using coated bullets, but some don't see much effect in that area. I can only say that for the purpose of shooting service rifle matches, I never saw any practical difference except the substantial metal fouling reduction.
