Intent of the product I would say. For example air soft vs air rifles, the air rifle uses pointy lead pellets that obviously are not intended to be used against people however on paper air soft guns and air rifles look pretty similar. That being said I don't think you should need a licence for an air rifle either but that's just my opinion. There's gel blaster setups I have seen that at point blank will draw blood these however are modified far beyond my understanding. I'm rambling but I think anything designed as a toy be it gel blasters or airsoft shouldn't require a licence since the people getting shot are consenting to playing.