Morning,
My other hobby is collecting and shooting vintage firearms and granted, the laws of this land regarding firearms can be a legal minefield. For example:
I have 4 different variants of the .577/450" Martini Henry. These are described as an "obsolete calibre" ie Section 58/2 by the Home Office. I can fairly easily buy all the components required to live fire the Martinis but would be breaking the law by doing so. I do shoot one of the Martinis as it has been entered onto my Firearms Certificate and its status has changed from obsolete calibre to full blown firearm. The 3 Martinis that are not on my certificate can hang on the wall above my fireplace without causing any concern but the one on ticket has to be stored in a vault although there is no mechanical difference between them!
Also, Black powder is classified as an explosive that requires a separate certificate to buy and transport and yet another certificate to be legal to store it (in any quantity) whereas, the nitro powder that is used in all my modern firearms is classed as a propellant and is not subject to any of the restrictions that its black cousin has.
I have found an example very similar to your cannon
[size=78%]http://www.gunstar.co.uk/not-deactivated-winchester-repeating-arms-company-usa-10-gau/Deactivated-Guns/902851[/size] and it clearly states "Obsolete/Antique, Section 58(2)"
It is perfectly legal to own as long as you dont try to fire it or start collecting the various components to make the ammunition for it (intent)
I would seek the clarification of your local Firearms licensing dept at police hq and read the relevant Home Office guidance notes to cover yourself but please be aware that the HO guidance is just that, guidance, and each individual police force can and do interpret the law differently.
These folks sell the 10g blanks
[/size][size=78%]https://www.shootingandscuba.co.uk/store/eley-10-gauge-yachting-blanks-pack-of-10[/size] and state "no licence required". I have on a couple of occasions brought ammunition from them in the past and the owner seems to be a very knowledgeable chap-you could always give him a call, stating your predicament and see what he has to say, just dont take any answer as gospel.
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