I have posted in past of wanting an original unmolested Hepburn and now I have one. Believe me it is untouched to the extent it was NEVER cleaned and the bore is as the seller described it "a sewer pipe". Honestly it has been a long time since I have seen a barrel this bad. It is an early gun with the following markings under the barrel 40 2 1/2 (40/70?) s/n 4876 N.V. 32 and ST. Can anyone decipher this for me? Other than the caliber stamp I don't know what the rest means. Another thing is the rim recess measures .580 dia. much larger than the brass I use for my 40-50 and it is a straight wall chamber which is pristine. I said I would never do anything to an all original but if I want to shoot it and I do I must now decide if I want a rebore or liner otherwise it is a paperweight. I got it at a reasonable price so I can afford to do the work. Advise is always welcome...


JMH

Depending on your expectations, I would clean it as well as possible, including something like JB bore paste and possibly lap it.

This was an unlikely accuracy candidate, I have never figured out the actual cartridge. .306” Groove, 32-20 case cut down to .600”. Stevens Tip up sporting open sights. Hard to keep 5 shots on a sheet of paper before cleaning, 1.5-2 MOA out to 75 yards after cleaning, bore paste and a light lap.

Photos are of the same spots in the barrel before and after. If you’re trying to keep it original, it’s worth a try. Lots of lube and hard, well fitting bullets really helped.

No expert here but those heavy chunking and stripped deposits spell lead and lots of it. Once lets starts to bed in the bore it has a tendency smear up near the chamber mouth and further from heat. Then as it beds or builds it attracts more lead. Lead likes to stick to lead. This looks beyond JB and even lapping. One of the old methods that really does work but may be difficult to find and is hazardous, is mercury. Mercury dissolves lead into an amalgam.

Cork the muzzle tight, pour and fill barrel, let stand a day or more outside in well vented area, then dump out mercury in a tight safe container. Insure you have good gloves and skin protection and a good organic vapor rated face mask. Mercury is toxic for sure but handling like this is very doable.

I would at the least go into the bore with a stiff brush over some white copy paper to make sure I'm right at the particles on the paper are lead. 
The pitting is another matter

Rick

Turps is a good choice as a preliminary approach but you need to find REAL distilled gum turpentine.  This is not easy trust me. My earlier boat building businesses relied on good quality distilled turps. It's hard to find these days and very expensive. But turps can penetrate the lead to steel surface bond and promote breaking it free. So good idea and worth starting off with. 

Rick

Thanks everyone. All I had on hand was good ol Hoppes #9 and a stiff brass brush. I alternated scrubbing and patches but gave up. The patches that came out were pure red rust. I looked for traces of lead but found none. I have delt with rusty barrels before but this one is one for the books. I do have some kroil on hand so will give that a try. Don't think I will give the mercury a go. Probably couldn't find it anyway. What is strange is the chamber is pristine. Not a spot of rust. I purchased it from a gent in Florida but didn't think to ask him if he knew the guns history or where he got it. He told me he had several. If I can get it to clean up at all I am going to shoot it just for the heck of it. Maybe I can hit the ground if nothing else... By the way does anyone sell replacement screws for it. A couple of the tang screws are really banged up.


JMH

If it's half as bad as you describe, you have nothing to lose. After trying any or all the above, lube a tight fitting patch, wrap it with 4-0 steel wool, lube that, then run it through the bore. It should be tight enough to require a good thump to get started. The steel wool will grab the lead and drag it out. 
Before you all set me on fire for running steel wool down the bore, remember it's already trashed. 
Also remember that everyone scrubs on bluing without a thought and without a scratch. 

Might check on Gunbroker for screws  as there is someone who offers screws for Hepburns on there think he also offers them on eBay , I don’t know his name maybe the same person Ranch 13 is  referring to , does anyone else besides me want to see some pictures of the rifle being discussed. 😁 JR

Using Mercury, you really don't need enough to completely fill the bore.
Any amt placed into the bore and then cap the open end or course.
Simply turn the bbl back and forth to run the Mercury up and down the bore of the bbl.
Just  few back and forth motions will do it.

It will cling to any lead in the bore and amalgum with it.
Keep doing the distribution technique a couple times a day for as long as you like.
Then carefully pour the lead out of the bore (careful it runs out very quick as it's heavy and can overshoot a container not  big enough.
Any lead that the exercise has loosened will come out in that merc.

Scrub the bbl and check. If needed, repeat as many times as needed.