Pitting....the barrel is 70 plus years old, clean it and shoot it if it works well dont worry about it, from what you stated it should work just fine for its intended purpose.
What kind of shape is the rifle in? Does it have clean original wood with factory but plate and are the sights in original condition? If it is a complete clean MOD70 in 375H&H and shoots ok enjoy it as a rare collectible and leave it as is.
Definitely pitting. You can now see the actual pits now that you removed the corrosion product that make the tiny 'bumps' at each pit. I agree with J.E. above, leaving the copper fouling in the barrel may be the best route. At least it will somewhat fill in the pits to a degree and make the surface less rough.Externally it looks in very good condition, the stock was original, it has a red butt from Winchester. It was cracked through the mag well between the action screws, I'm guessing because back then crossbolts were not in fashion. I had my gunsmith fix the crack, I think he injected epoxy and clamped it. I put it away and had a fiberglass stock made which I use now that adds some weight and it shoots well as you can hold with open sights. The rear sight is dovetailed just forward of the chamber, and was a bit loose, I put shim stock under it and it's tight again.
It came with only two holes in the front of the receiver, I had two put in the rear, but it's impossible to mount any bases on or over the receiver, so I just put the grub screws back in.
I've done one night of a Hoppes #9 soak, I have to go get and some Boretech.
The marks are clearly some sort of corrosion, it is definitely pitting.
This is the chamber, with fine pitting -
View attachment 173742
This is towards the end of the barrel
View attachment 173743
This is just ahead of the chamber
View attachment 173744
I'll update after a few days of Boretech treatment.
I would switch to JB bore paste at this time and knock those bumps out.After two night in Boretech Eliminator.
The ends of the barrel have cleaned up from the brass.
The mid section still has some brass fouling, one more night should fix that.
Some of the liquid pooled outside the muzzle while standing overnight, I now have a brasss front sight that I never knew I had before.