Jeff, If your barrel is not chrome lined... I don't think 870's are, then your problem is probably lead or most likely wad plastic build-up. A good cleaning method is to use Hope's Copper Solvent. Saturate a loose patch & soak the bore with it. Then stand the gun on its muzzle overnight,put a thick patch under the muzzle. Use a tight,dry patch to finish the bore. It will take out lead, plastic,& powder fouling.
If there's still something in there after this treatment then, you better find a bore scope to determine if the barrel is damaged.
Luck to ya,
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Ralph in In.
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