Welcome.
I see the .260 as somewhat under powered when compared to its perennial rival, the 6.5-284. But of the two, I choose the 260 because of bore life. And now, I see the 6.5CM as being somewhat less (which is somehow being represented as more). And then, there's the 6.5x47, which I find as quite intriguing despite even less case capacity. I've seen it do remarkable things at 300yd, and wondered whether it's a genuine 1000yd capable cartridge. Looks like it is.
As for Varget vs H-4350, I agree, now's not the time to change horses in midstream.
But downstream...? According to the Hodgdon site, in the 6.5x47, H-4350 produces the highest listed velocity.
200yd vs 100yd testing for LR chambering demonstrates vertical dispersion better than 100yd, while still keeping the effects of wind down to a manageable/understandable degree. That vertical dispersion relates to things a chrono can tell you, but I don't own one. My target is my chrono.
Almost any powder can produce exaggerated fouling up until the pressure level gets significant. Carbon fouling is (IMHO) what's left of kernel coatings after combustion. Traditionally, those coatings have been graphite, which is coincidentally a dry lubricant. Coatings are evolving, and my intuition suggests they may be in some way related to moly. When folks bring moly to the table (er, bore...), I consider it to be an added complexity in the bore velocity/bore friction/pressure/velocity equation. We already have our dry lube in the form of 'carbon' (graphite) fouling. Anything which reduces bore friction in turn reduces pressure, and in turn reduces velocity. Bore fouling status bears directly on velocity, and is why a clean bore needs fouling to deliver a steady POI. Bore pressure is better when it's up there (but not peaking), in order to ensure that the fouling is mostly, if not all, the residual kernel coating. If I thought moly was a valuable complication, I would mix it with my powder, not plate it onto my bullets.
If I sound like I like hot loads, I don't; I think they do only a few things, like burning up barrels and blowing up guns. Metal fatigue is reality, as is Mean Time Between Failures (MTBF). Velocity does not conquer wind, good shooting skills do that; and cranking up the velocity is just a tempting illusion.
Confused yet?
The complexities can be confusing until one takes a step back and applies the scientific method. Accept that there are complexities, but also that they work fine together until we start manipulating the individual factors.
Doing so with a method is called experimentation, without the method, it is just shooting in the dark. If I'm going to do manipulation, it will be about removing individual factors, and not about slipping additional ones into the melee. Removing a factor reveals the consequences of the factor's presence, and putting it back in confirms the conclusions. Once we know the consequences, with and without the factor, we can try substitution, and gain more insight. Reintroducing the original factor in combination with the new factor reveals whether they are compatible, i.e. whether the new factor helps or hinders.
I'm not talking about your issue right now, but about issues in general.
The point is that this stuff is a long game. The other point is that, like you, I am mindful of bore longevity. Maybe too mindful, but that's just me.
Keep your experiments down to altering only one factor at a time. It's tempting to skip the middle stuff, but if we do, we really don't know which alteration makes which outcome a reality. The time to combine factors (components) is at the beginning, and provides the whole foundation for the experiment. When the experiment fails to achieve progress, it usually means that the initial foundation is non-supportive.
Make your experiments statistically meaningful. Three round groups are occasionally thrilling, and statistically meaningless. Five round groups show coarse trends, but are not conclusive. Twenty round groups are very revealing, but hard on the bore. You have to weigh all of these alternatives. Once you 'have' your solution, test it rigorously before committing to volume ammunition production.
Stay the course, finish a sequence of test regardless of how intuition pesters you. Without a full data set, the experiment is flawed. Accept that barrels are a 'wear item' and that what you are doing is more about testing than about competitive achievement. Share your results and you will usually learn more from the sharing. In order to save barrels, you usually need to burn barrels initially. Careful/judicious test design and experimental discipline can keep that necessary bore wear down to the minimum.
The human memory being what it is, take notes; complete notes; and follow up your conclusions with a journal.
Greg
