Joab.
Justify - if one knows what they are talking about, the justification of the use of deadly force is the prevention of grievous bodily harm.
If one knows what they are talking about, one can find that BB guns, varying in type, are capable of producing serious injuries. Search the medical literature - it is easy to do at the library.
Thus, if someone is shooting at you maliciously with a BB gun, you have justification. If you choose another strategy, that is your decision.
In any situation, avoidance is better than gun fire - however, if it comes down to you being hit with the BBs, then you have justification.
That is simple enough to understand. The comment that folks want an excuse to shoot someone is rather stupid, BTW. That's your projection and attempt at cheap rhetoric.