Just a quick update on the progress of this thing. To answer some questions:

Just ensure that you can shoot it where you intend to, range certificates usually have limitation on M.E and/or calibre. .75 is quite a biggy
I shoot at two ranges - one in Worcester where the limits are .577 for breech loading, and .75 for muzzle loading (hence the reason I decided on .75") and 5160 ft lbs. I made an excel sheet to calculate muzzle energies (available for download here) which tells me that the limit is 1915 ft/sec, much higher than I anticipate. Tests on other cannons show me that 1000 ft/sec is much more realistic, so I will be well within that range certificate.
I also shoot at a range in South Wales, which has no calibre limit, just an ME limit is 7000 Joules (5162 ft lbs). I have on ticket a 1.75" cannon which just happens to be about the size of a golf ball.


I will sell you the insurance
One of the first things that we did as a club is go to our insurers to check that this kind of thing is ok - along with cannons. We are insured with the NRA, and after much discussion, there are no problems so long as the item is aimed, and is within the range safety certificate.


I have nearly finished the handgonne (but not drilled the touch hole, so it is still just a model), and was about to write it onto my ticket as a cannon - as I have a slot for a .75" cannon. After having a think about it, I have decided that it is a 'pistol', so have put in for a variation. The reason for this is that if it goes down as a cannon, nobody else can have a go - it is a bit of a strange one under the firearms act. As it will be controlled by one hand (the other one will be lighting the thing), I reckon that it's a muzzle loading pistol - which means that anybody at the club can have a go.

I am going to test it a few times first with a heavy load of powder and lead to make sure that it doesn't explode (and that my calcs are correct), and will then probably send it for proof - just to be secure in my own mind. Pics to follow as soon as it is on ticket.