Yes and it did state what it's for on it, could be possible there was a question of what type of gun it was for that got it rejected for the one that was stopped, sub 12 to UK should be fine declared and they should know to put it on the form if they ship here.
I did keep the form from mine but I can't remember where I put it otherwise I'd take a pic so you've got an example they could go from, in my case the original sellers didn't ship outside the US so it went via an Airgunner in California who boxed everything up and did the customs form, then sending it via USPS with a 2 week stop at Customs in the UK and nothing more paid than the initial stuff at the US end.