Here in UK, if discharges a projectile by means of a gas generated by explosive combustion, like, for instance, a primer, it is a firearm. More to the point is that this little item may well fall under the heading of prohibited fiearms on mainland UK by virtue of the fact that it is indisputably a handgun. In fact, it is actually designed to be concealable, so it way well 'fit' the bill as a 'concealed weapon', even in Northern Ireland where handguns are still permitted.
tac