AR2001,

One of my 500 Nitro doubles has had a scope mount base fitted to the rib
(it came with an Aimpoint scope) and this hasn't affected the regulation.
- it's not something I would do but I bought it like that.

A mate who owns a Berretta 9.3 x 74 has had NON factory scope mounts
added by one of our better gunsmiths and they work fine and haven't affected
regulation.

Technical question, do you know how much freebore these 9.3x74R's come with?
NO, I don't and I don't worry about these things.

Re ""I have also been eyeing off the Pedersoli, it has a "solid rib" whatever that means. I am taking odds with
myself that drilling and mounting a weaver rail on a "solid rib" won't compromise regulation, but what do
I know ......"

I don't know either - a mate has a Pedersoli but I've only seen it once.

I can ask him what he thinks of it if you like ?

500 Nitro