Most of the so-called “pen guns” are NFA Title 2 “Any Other Weapons” and must be registered with the ATF… not so the various iterations of the “Stinger” pen gun, because it must be reconfigured before firing. Because of this, it is treated by the ATF as just a normal handgun.
The Stinger is no longer in production and is something of a collector’s item, but it is a wonderful example of engineering ingenuity.
The decision to put a can on the pen gun that is bigger than the gun itself is a very amusing one, and I suspect that it was driven by curiosity more than anything else.
Despite having no practical use, I suspect this would be a very fun novelty gun to own.