260rem,

I think the only handicap is a minor one in the field.

A real handicap might be if you ever hunt Africa where a 9.3x74R might not be legal for some dangerous game.

There isn't a lot between the two cartridges.

I like your idea of a drilling a lot more than a shotgun designed for slugs or buckshot. A 9.3 plus two brenneckes, if needed, would do some damage on scrub bulls and buffalo at closer ranges. An ideal close pig gun too.

On the scope question, I do not like scopes on a shotgun if shooting shot. A good set of claw QD mounts would be needed.

However a double rifle with shotgun barrels is ALSO not a bad idea.