Generally when shooting singles with an O/U only the bottom barrel is used for reasons of lower felt recoil.

Maybe I'm wrong, but isn't this what's always been called riveling?

Destry

Destroy I looked up reveling on Google and found a few articles. It sounds like it could be the same thing.

Sorry to say, but it is not. The original scenario is the functional story. There will always be other "expert opinions," but these pimple bulges are what they are: Real.

But there is little need to concern yourself with them as long as your cartridges are securely crimped.