Despite what the Police might have said a few years back, Ive spoken to a friend of mine in the UK police force who is a firearms officer and he feels that you should hand it into your local larger branch of the police assuming there is one because it is actually capable of firing a blank cartridge that means it could possibly be used, altered or seen as an offensive weapon so basically take it down to the station and force them to accept it and give you a receipt make sure that with any cartridges to go with it you make sure the item is empty and the cartridges are in a second bag and not with the main device and make sure that you cover put both of them in separate bags , better still, if you do have a cartridges, give them to another person to go with you.
I think he said you might actually need a license to own a new one of those nowadays certainly in the UK I know here in Spain you would not a firearms license but something similar for construction because they are capable of firing fixing directly into a very thick RSJ's and reinforced vibrated concrete.
He said if that sounds like too much hard work then basically disable/break the mechanism that fires the cartridge and toss the item in with the metal scrap, that's easy enough to do but just by drilling a large hole through it or bash it a few times with a very large hammer. However, still hand the cartridge is into the police because they do contain an explosive will be at minor.
They need to be disposed of safely.