I have the option to acquire a nice side-plated blne 20G sxs, with 28" bbl. and English stock. I hesitate, not that the bbl. rib is raised and flat on top (not concave), but because upon using a straight-edge from the bbl. front-bead to the breach-tang (that which the Greener cross-bolt bolts into), there is approximate a 3/32" drop at the top surface......so, the top-rib surface/plane looks bowed :(, along its entire length.

How might this affect my or its performance?

John

We're only responsible for the effort, not the outcome.