Probably most of them too Jim.
Gun may be old but can you prove the date of the conversion?
We could not for instance have a firearm made for an obsolete cartridge today, age comes into it as well.
If the history of the piece was ok then it could be owned but not ammo. (sec 7.1).
If you wanted to fire it then it would have to be permanently stored at a range that is licenced for sec 7.3, I think there are 3 in the UK and storage fees are very high.
I don't think .45 short/long Colt is on the obsolete list which is what most conversions seem to be.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane