"Would you consider it safe to put a few rounds down range to clean the barrel out a little more?"

Curt, in a word: NO.
Painting metal parts with nice thick silver paint is a standard dodge for decoration pieces to fill up the rust pits on the outside. And you say yourself that the bore looks like a sewer pipe. Do not try to shoot it clean!

There are 2 basic types of sewer-pipe bore:
1) Plain filthy. In which case: clean it!!!
2) Rust-pitted. In which case: bin it!!
How to tell the difference in a few seconds? Push a tight-fitting cloth or felt patch through the bore. If it comes out filthy, you may well have type 1 ... keep on cleaning.
If the patch comes out torn ragged, you have type 2, and any further effort is wasted.

I have restored several type 1s that looked at first to be junkers. It's just elbow grease and patience. But a bore that shreds patches is scrap.

Patrick