The turrets on Belfast can still traverse as well - I've noticed they do shift from time to time. One of my old man's acquaintances is a volunteer aboard her and helps maintain the guns and their traverse/elevation mechanism by greasing them and so on. Wouldn't surprise me if the main battery's still live - some of the secondary battery is definitely in full working order and apparently this summer they're going to fire blanks from one of them!
While wandering round the ship I did see a No.4 secured in a side cabin by a pretty hefty chain, along with what looked like a couple of Stens. I wonder...!
Well the largest rifle calibre (on pure bore size) i have is .577 snider.
I do a bit of re-enactment at a restored victorian fort in plymouth every few years and on the managers firearm certificate are such nice toys (which i get to play with some times) as 2 7" armstrong guns, 4 32 pounder breechloaders, 2 32 pounder muzzleloaders, 2 15" mortars, 1 75mm field gun. Also stored there is a 15 pounder field gun but on a private firearms certificate.
There are only a few security measures required like keeping breech blocks for the armstrongs in the small arms armoury.
I also help out crewing the ww1 Monitor HMS M33 currently dry docked in portsmouth, its 6" main guns and 12 pounder anti-aircraft gun are all live and on ticket but cannot be currently blank fired due to possible blast damage to HMS Victory. The Manager does have bits for a 12 bore sub calibre insert but it is the wrong mark for its guns. But this didnt stop me from shooting a Blank firing a Lewis gun from its deck!