I'll answer this from a claims adjusters perspective (been doing it for 20 years now).
The questions is: if you shoot at someone and hit them but the bullet over penetrates and hits someone else, or you miss and hit someone else, can you be held liable?
The answer is: Maybe.
I don't mean that flippantly. It comes down to whether or not the person injured, or their attorney, can convince a judge that they have a reasonable chance of prevailing if they sue you. If they do, then a judge will likely allow the suit to move forward. You will then go to court, both sides will argue their case, an a jury will decide if you were negligent, if you are liable, and if so, what you are liable for.
The second issue would be determining if you covered for this collateral damage by your insurance company. My answer to that is mostly likely yes. The reason is because it comes down to intent. You intended to shoot person X, but you hit person Y. That is unintentional, and likely covered by your insurer.
I handled a claim many years ago where someone shot at an ATV that was trespassing and accidentally hit the rider. The was covered because they made a convincing argument that they never intended to hit the rider. Hitting the rider was negligence.
I've handled a number of suicide claims where someone kills themselves and ruins their car or ceiling in the process. The damage to the car or house is usually covered since it was unintientional.
Rarely will an insurer actually apply an intentional act exclusion to collateral damage.
If you ever get sued for collateral damage, you can be sure the lawsuit will allege negligence. The allegation of negligence in a lawsuit will trigger the insurer's duty to defend you (even if the suit is baseless), and if you are found to have been negligent, the insurer will have a duty to pay.
If you are sued for collateral damage due to a self defense shooting, you can bet your bippee that the attorney suing you will also sue the BG you shot (or shot at) for causing the situation that force you to shoot.
