I happened into an old Remington model 12 reasonable, and the barrel may just be full of 103 years of lead fouling. Patches have shiny metal flakes in them. Lead removal suggestions?

I have a 22 martini that has Canadian ownership marks on the receiver. This particular barrel actually had lumps of both lead and copper fouling on the tops of the lands. Cleaned it, cleaned it some more and finally just kept the barrel wet with hoppe's. Used brand new 22 bore brush that for some reason really fit the barrel tight. About a dozen strokes finally showed some flakes so kept at it. After about 20 minutes patched out the barrel and the lumps were no longer there. Had the same problem with a 1915 vintage 1895 win in 30-40 krag. Had what looked like lumps on top of the lands. 8mm bore brush and no longer there. That one had been soaking with hoppes also. Frank

Get a pure copper scouring pad (take a magnet with you to check, some brands are copper plated steel, pure copper wont scratch your barrel but steel will) unwrap a few strands and wrap them around a bore brush and scrub away. Should only tale a few passes. You can use Kroil or any bore cleaner as a lubricant.

Long ago Creighton Audette introduced me diamond paste on a patch wrapped around a worn brush after showing me I wasn't getting my barrels clean. Not for everyday use, but great on severely fouled barrels because not many passes with the cleaning rod are required. Remove with degreaser on a patch. Unfortunately Hoppes ain't what it used to be.....

This is pretty standard. First, do not use anything in the way of lapping, abrasive to do this, as you're likely going to hurt the bore.
Use something purpose built. Get some IOSSO bore cleaner and preferably a couple of their hard nylon brushes. This stuff is purpose built for lead/carbon. You need to brush, patch it several times, you have layers of both lead and carbon. The patches will be charcoal black and get progressively lighter. After a few cycles, use a patch wrapped tight around the brush, soaked up good with the cleaner and you will get to bare metal in short order without hurting anything.

I'm going on orders right now. Left it soaking in Ballistol.

Patches came out black, then clean ones til clear. Finally found a .22 Tornado bore brush and the patches are coming out brownish black again on a patch just goopy with Ballistol. Soaking for 24 hours, then dry patches tonight.

Have you tried shooting it at this point? My M12 has a bore like a sewer pipe, but is surprisingly accurate. Every time I start thinking about having it lined, I shoot it, shrug my shoulders, and put it away.