I can’t say any of this comes as a surprise
You are of course completely correct. If you stand your ground, politely, you will eventually get your airgun. Expect to receive many threatening letters and possibly have a trip to court before then, do not expect HMRC ever to admit they are wrong, and when they are shown to be wrong do not expect them to do anything different next time
Are you n BASC? If you are I’d give them a ring.