The DTI publish a fact sheet on faulty goods and the Supply of Goods and Services Act 1982. In this document they say that "Whenever goods are bought they must "conform to contract". This means they must be as described, fit for purpose and of satisfactory quality, if not the person/firm/supplier are in breach of this "legally binding" contract so quote the above and with a bit of luck they will shit themselves and hopefully "get their fingers out" and get your gun back.
Hope this is of help
Coney.