Accidental discharge is the question. With a bone stock gun, you will be liable, as somehow during handling of the gun it discharged. Depending on the circumstances you could face criminal charges, but for injury or death you would certainly face civil charges. The civil attorney suing you may also go after the gun manufacturer (deep pockets).

With modifications to a gun, the first question would be "could those modifications have made the gun less safe than the stock gun? Was the cause of the discharge related to any of the modifications?" If the answer is yes, or could be yes, to either of those questions, then you could criminally face some type of negligent charges (you did not intend it to happen). If you are sued in a civil court, the modifications will be used to to convince the jury to award higher damages, as the gun was "not safe and was more likely to discharge". With modifications to the gun, the gun manufacturer would most likely not be included in the lawsuit.

The above is for accidental discharge. If you shoot someone with intent (you thought about it, took action, and completed your intent), then any modifications would not matter. It is like someone said, if you had a trigger that was so light it would fire if a fly landed on it, it still would not matter. It was not an accidental discharge. You had intent (thought about shooting the person, even for only a second). You took action (you aimed the gun and pulled the trigger). You completed your intent (the gun fired and the bullet hit your target). In this situation, depending on the circumstances, the charges could be nothing (a self defense situation) all the way up to fist degree murder (you planned it and carried it out).

Even if this shooting was determined to be an obvious SD situation, and no criminal charges are filed, you still will likely be sued by the victim, or the victim's family. You could be totally in the right by defending yourself or family, and you will still be sued. Most people are not aware of the costs of defending themselves in a "good" shooting. As we all know, in our country you can sue for almost anything. The costs for defending yourself falls upon yourself, with little chance of recovering those costs.

Bob