And it would be regulated as a firearm in the UK ("a lethal barrelled weapon of any description from which any shot, bullet or other missile can be discharged"; section 57 (1) Firearms Act 1968). There is also a specific list of prohibited weapons in the same act which are firearms even if non-lethal.
If the subject of skirting the weapons laws in Germany is of (presumably academic) interest to you, the Slingshot Channel on YouTube is a must-see.
I watch it for Jörg's cackling laughter and facility in ginning up impractical weapons, but it's good for the above reason as well.
I'm reasonably sure that anything that can fire projectiles with more than a few Joules is a weapon in Germany.