It depends how far you're talking about when it comes to a licence. If you're talking about a full PAL, that might be a bit of overkill for a BB gun. For licensing, what I think would be more appropriate would be something like the two-tier model that's used in the U.K., where you're educating and vetting that person ahead of time, before they purchase, by having them join the association and having to, more or less, complete a test so they know the rules of what they're supposed to do.
If that would be considered a form of licensing, and you'd get an airsoft licence after that so you could purchase normal airsoft equipment, I think that would probably be more in tune with—