Rose might be able to help with that. At WWIIRESW we have an insurance policy that about 70 of us buy in to for Public Liability, personal accident and kit insurance - https://wwiireenactmentsouthwest.co.uk/oxwall/groups/8
Hiya Liskeardwarden
No law around it as it does not fire projectiles! however you do need to be a reenactor with insurance as it is a replica firearm if you don't want it to have orange painted on it!
We run ours off a large oxygen bottle and a propane bottle - these fit into the sidecar but we also have boxes so it is able to be put in a trench.
It actually sounds ok, I've included a link to our you tube video - and has been really reliable as long as you play with the mixture a little at first which can vary depending on the temperature. There's no limit to how much you can fire it - except for heat build up!
Also - don't stand in front of it haha!
If you are coming to Armed Forces Day at Davidstow you can have a go on it :-)