Would using a low melt casting alloy like Cerrosafe (there are others) down in betw the ribs help?
They melt at temps below that of 200F. They are heavy like lead.
When cooled off from the casting procedure and given a certain amt of time (1hr IIRC) the casting returns to full size in the cavity (no shrinkage). So it won't be loose and shaking around in there with use.
Warming the bbl set before pouring makes them fill nicely. Just like casting bullets but at 200F or a bit less.
They weight about the same as lead and the exact weight is available with some search.
If you had a loose rib issue, the Cerrosafe or other low temp weight metal could first be removed before making repairs by simply placing the bbls muzzle down and reheating the
bbl's up to around 200F again.
It will melt and flow out especially with that small rib hole upstream at the forend hook.
You can even place them in a boiling water tank and it'll melt and run out.,,that's how I generally 'melt out' a cast done with the stuff.
Let the water cool and the metal hardens and is reuseable,,but that's not the primary concern here.
