We used to use diluted muriatic acid (about 10% acid IIRC) to etch small parts,,but we did it with parts being warm as just pulled out of the hot water tank.
They etched instantly then a quick dunking back in the water and back out and a coating of Express Rust blue on them and off you went.
This etching was just done once, before the first coating of bluing soln was applied.
If you wanted to try an etch, I'd go with the ferric chloride as in a damascus finish etch.
A quick in and out cold dunking with the bores protected. Let the entire assembly etch,,tubes, ribs, ect and rinse off. Then procede to bluing.
Certainly worth a try at this point.
That's about a last resort.
If you don't want to try it, then the Cerakote idea comes into play I guess.
Those ribs may even be a rust resistant alloy as you can get them to rust a bit, but it comes right back off.
Modern guns,,what a pain!
