Well, basically if it’s dark it’s going to pretty much stay that way. There’s no way of taking pits out of a bore.
Letting the bore soak in Kroil (which is a penetrating oil) may loosen up things.
Also, this IMHO is the one and only instance where a stainless steel bore brush can actually be useful. They are very aggressive and can damage a good bore, but if the bore is in bad shape already, they’re worth a try. Basically they are good for scrubbing out the pits (I don’t mean removing the pits – which you can’t do - but just getting the rust out of them).
At some point, also using an abrasive such as J-B Bore Cleaner or Remington Bore Cleaner may help polish things at lest a bit. However, they are pretty mild.
As mentioned by Blankewaffen98, a pitted bore can still be pretty accurate. (And it’s not like Moisen Nagant’s are target rifles anyway. [:D])
However, cleaning is going to be a chore. I’d recommend you get some Sweets Bore Cleaner for routine decoppering – it’s quite aggressive and should speed up your cleaning.