I have a new to me Winchester M70 Left Hand rebarreled to .375 H&H that has kept me busy for many weeks trying to find a load it likes. It is the fussiest rifle I've ever worked with. I wanted a standard load shooting Barnes TSX 270 gn but, no way. Wouldn't shoot those. It showed much more promise with Barnes TSX 300 gn so work began to find a standard load with this bullet. I worked through the main suitable powders I have - RL15, RL17, W760, AR2206H ( H4895 ), AR2208 ( Varget ), AR2209 ( H4350 ), AR2213sc ( H4831 ), IMR 4064, RL23. While W760 was best overall there was little consistency. W760 loads would shoot very nicely one day, only to disappoint the next. I was starting to despair and began thinking maybe a new barrel was in order. But having one made here in New Zealand is no straightforward undertaking. Somehow I would have to press on. I was desperately searching the Net for ideas and answers and noticed reports from guys using Staball 6.5 with satisfaction. Unlike many hard to obtain powders here currently Staball 6.5 seems to be in good supply. Having nothing to lose I bought some, found some load data at Hodgdon's site and made up some loads. Well, right off the bat I saw Staball 6.5 could be the game changer. Over several range sessions I narrowed down to a final load my rifle likes that shoots accurately and consistently. This is the load I will take to Zim in August.
The pic shows POI at 200 yds. At 100 yds the POI is on the top edge of the target square. OK, the bullet holes aren't touching but this degree of accuracy will easily work for game within reasonable range.
I started this load development in October '22 and only settled on this load a few days ago. Quite a mission compared to my Blaser .375 H&H which usually shows me a decent load in about three range sessions.
Hodgdon's load data shows a maximum charge of 80.00 gns Staball 6.5 with a 300 gn bullet which may even fit under the Barnes TSX 300 gn. With my charge there is still a bit of room under the long Barnes bullet ( my COAL is 94.00 mm, which is long ) but I'm not inclined to use a heavier charge. I'm happy now.
In appearance Staball 6.5 is much like W760, being a tiny, flattened spherical flake and I have learned to pour it very slowly and carefully from the container into my powder hopper and trickler. At first I didn't notice but the stuff seems to migrate everywhere. When tidying up my loading bench I have found kernels scattered everywhere over both the bench and concrete floor. I need a system that contains it better so I'm not wasting it.
Another good characteristic of Staball 6.5 is the reduction of copper fouling which I found to be as claimed. Actually, I am pretty impressed by this. My barrel is noticeably copper fouled by most of the above powders, particularly along the central area of the bore and I was scrubbing this out after around 50ish rounds. Not due to accuracy loss but because I did not want fouling building up further. With Staball 6.5 even after about 100 rounds through all I can find by borescoping is one small copper streak halfway down the bore and nothing anywhere else. It's the best anti-fouling powder I have used to date.
Anyway, hope this is of interest to guys looking for an additional powder for the .375 H&H

I don't know why you would be thinking a new barrel might be in order after failing to find an accurate load with the range of fairly normal powders you tried, I would be ditching the Barnes bullets which are most likely the problem and go with something else. Barnes are not the begin all end all in bullets, there are plenty of other excellent performers out there including cup and cores. I wouldn't bother with a bullet that only performs with one powder, a lessor known one at that despite current availability here in NZ.
As for copper fouling, again ditch Barnes problem likely solved.

Have you tried other bullet and powder combinations. What happens if your ammo goes astray enroute to Africa.