Honestly, I don't believe a BB gun can kill. The Dude, I'm a bit skeptical that your BB gun can burst through someone's head (but if you prove that it can, then kudos to you).
See, the most harmful BB gun out there are the late-dated ones. I own a Daisy air rifle, modeled after a Winchester western. Its the kind where you can feed a 100+ Copperhead BBs into the chamber but you have to pump the stirrup to compress the air. After several pumps, a BB gun can go through a soda can but not through a human skull, even point blank range.
Other BB guns, like the Classic Army and Tokyo Mauri that are modeled after realistic guns use either gas cartridges or electricity to fire the BBs. This means that the BBs achieve a powerful velocity. However, they can only light plastic BBs that do diddly-squat. You might get a sharp sting but that's about it. A paintball gun hurts more than an AEG (Automatic Electric Gun). I own a Tokyo Mauri Mini-G3 and it barely goes fifteen feet. Additionally, I've used spring-action and gas-action BB guns and they only sting.
So BB guns, both the new ones (AEGs) and the old ones (Daisy air guns) cannot kill. If you're wearing proper equipment, like eye protecters, than you shouldn't have to worry. Of course, there have been some cases were a shot hits the eye and the person dies of hemorhaging without immediate care. But, that's always a fluke.