Mini cannons and the law

[$harp$hooter]

I've done a search and read numerous threads about cannons and mini cannons but wanted to clarify something in relation to s58(2) and mini cannons.

I recently saw a pair of 13.5cm brass cannons for sale on evilbay with a bore 6mm in diameter. The seller stated in the listing that's these we're made approx 1920 an that they were live as the touch hole goes through the wall into the bore making it able to fire.

Now I missed the auction end and didn't bid but it got me thinking about their legality.

Withing s58(2) any muzzleloader is classed as 'antique', however seen as they could assumedly be filled with BP and fire a projectile, would this mean they then fall as a section 5 prohibited weapon due to barrel length or a section 2 requiring a SGC?

If someone could clarify if my thinking is right or correct me to get the right answer that would be great. I'm new to BP so trying to understand the law.