I believe what you have there is a Remington Rider No. 3 shotgun.
87,000+ made from 1893-1903.
You got a deal at $50, it's probably closer to a $125-$150 value.
Your bore looks more like buildup than damage. I use a copper pot
scrubber pad on a rod and a cordless drill, with a liberal application
of Kroil or Ed's Red for bores gunked up like that.