Yeah, but "dark" bores can come from many sources, not only corrosive ammo. I've had many clean up just fine, the crud being ony petrified cosmocrud and dirt. Some ARE rusty, and may or not clean up. Plus some NEVER clean up, but shoot fine, and some "shiny" ones don't shoot well at all!
AIM likes to say "Bores not new" or some such disclaimer on Mosins, and I appreciate their candor, but others you don't know what they look like until you get them off the BBT.
I first try to clean mine with a lot of solvent and a tornado brush and some elbow grease. SOMETIMES this is all it takes.
On REALLY bad ones I use the "Hot bore" method. Take it out and put about 15-20 rounds of surplus FMJ down it as fast as you can work the bolt and load it, until the barrel HURTS and leaves a mark if you touch it, not merely warm! (TRUST me, 20 rounds rapid is all it takes, those handguards are there for a reason!
Then IMMEDIATELY try the tornado brush and solvent again, and stay at it. Repeat the hot bore as necessary or as ammo permits.
MANY times they will be shiny and sharp after that...
If not, shoot it for accuracy, if not accurate enough, sell it and buy another and repeat until you get one you like. If it shoots well, KEEP it, and ignorre the "darkness..."
I find M44s to be more accurate shooters DESPITE bore condition. M38s show more variance, and may take shooting many before you find one that shoots as well as an M44. I've never figured out why, except that all of the recent ones have been counterbored at the muzzle. Most M44s have not.(?)