Anything to this? or is it much to do about nothing?
TLDR: yes the risk is real. If and when it happens, it certainly ruins the ammo performance the same way variable neck tension does. In the real world, we manage risks in terms of likelihood and consequence. This problem is well known but difficult to reproduce in labs, and the consequences are bad enough that we design for it with stuff like sealants and "other" means.
A very long time ago I was visiting a well attended match. It was either a big state match or a national one. I think it was a silhouette match but it was more than 40 years ago so you must forgive me cause I don't remember specifically the match, but one of the things that stuck in my head was....
I was still pretty young and immersed in the defense world, so by then I had a pretty good ballistics background in that respect but they still encouraged me to visit small caliber stuff since my world was mostly the heavy stuff.
I had already been introduced to a young man who was an awesome shot so I felt comfortable asking him what he was doing. He was bumping the seating on his rounds. He explained it to me and I was wondering why that had never occurred to me before. He had a tough schedule and had to load his ammo long before the matches and did this just as he arrived. He showed me and every once in a while one would "pop". He would segregate that round for a sighter or reject it if he had enough.
In the military world, most small arms ammo has a crimp, neck sealant, and "other" that prevents this issue, but since we were in a different context what he was doing made a lot of sense. I have learned a lot from folks with no formal credentials, just like I have from the heavy hitters at the national labs and defense corporations. I learned a lot from that guy and consider him a friend to this day. His name is David Tubb and he doesn't have formal letters next to his name but his CV has many championships in several shooting disciplines.
If the issue is "cold welding", does a "barrier" (graphite, HBN, etc...) help with this? What about military surplus?
The way we look at this risk in the defense world is different than those of commercial ammo. We look at things in terms of likelihood and consequences. As a result you will typically see the standardized bulk ammo has a pitch sealant in the necks, a crimp, and sealed primer. We also store ammo according to specifications to protect it from water or other ionic risks.
To judge the performance of those materials you asked about, you just have to compare the conductivity of the bare ammo to samples of your ones with the "barrier". If you can measure the resistance across the bullet to the case neck, you will find that it takes a lot of barrier to cut off that conductivity path. If there is good conductivity and we introduce stuff like water vapor, we open the door to the problem since copper likes to trade electrons pretty well. I'm not saying stuff like MoS2, HBN, Graphite, etc, don't help at all, but a dusting of those isn't going to prevent conductivity or lock out the water vapor.
If you want to protect against this risk, keep your ammo in US Mil-Spec ammo cans while you travel. If you ammo gets wet while you carry it in the field, consider putting it aside for long term testing and see how it tests after you let it sit a few weeks after you get home. As always, YMMV
