If you have any Wipeout foaming cleaner, it works great on lead fouling, it just takes a couple days to soak, then push the fouling out with a patch, I had a terribly fouled muzzleloader, spent hours with Shooter's Choice and a couple other solvents, then read on the Wipeout website that it destroys alloys in lead that break it down, let it soak for a couple days and the lead came out in flakes!
Once you get it clean, load the biggest diameter bullet you can chamber, you're most likely shooting undersize bullets, I've had the best luck with .459" or .460" cast in my 45-70s. To check it, see how your current bullet fit in an unsized fired case from your 45-70, if it drops right in, it's too small.
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Tim