An old friend and his son were over here last night, and I'd gotten a double rifle out for them to see that has 30" barrels. The boy wanted to see the lion that was engraved on that particular rifle, and when it came out of the vault he exclaimed that it looked (to him) like the rifle from the start of 'Gentlemen.

It's been at least a couple years since I've seen the film.

Continuity hasn't ever been Hollywood's strong suit regarding firearms - and I think that 'Matilda' might have been represented by more than one rifle - and more than one action type.
At the beginning of the film, where the character Connery plays introduces the rifle, it appears to have very, very long barrels. Longer than 30" if I recall, and I've banged away at Google image searches and web searches with no fruitful success.

If I recall, we have at least a couple guys from the Hollywood scene in this group. Maybe they know something more about that particular rifle.

Cheers
Tinker