The only way a modified gun would have any bearing on a criminal or civil case would be if it was accidentally fired. All my handguns have modifications that allow me to shoot better. I am not the least bit concerned about legal problems. The guns are easier for me to shoot and shoot more accurately, while remaining safe to handle.
Accidental discharge is the key difference. 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 likely face civil charges. The civil attorney suing you may also go after the gun manufacturer, (unsafe design, and 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 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, as your modifications made the gun less safe, not their design.
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. If you had a trigger pull that was so light it would fire if a fly landed on the trigger, 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 first 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 would fall upon you, with little chance of recovering those costs. Many people have gone bankrupt, from attorney fees, defending themselves from a civil trial for damages. I am not saying to not shoot because you may get sued. I would shoot to save a life, but I know what the results of that shooting could be.
If you shoot someone and intended to shoot them, it would not matter at all how you modified the gun. NOT AT ALL. You had the intent to shoot them as you thought about it and decided to shoot. It would not matter if it had a "hair trigger"? It would not matter if you eliminated the manual safety? Those modifications had no bearing on the shooting. You aimed the gun and pulled the trigger. Whether the trigger pulled easier, or you didn't have to release the safety first, it wouldn't matter.
That is why I am always thinking about different scenarios involving SD situations. What would I do, how would I react, what would justify my pulling the trigger. As a LEO I always had the mindset that I would never shoot anyone unless it was a life or death situation, even though we could legally shoot for felony situations. I would only shoot to protect myself or to protect the life of someone else, who was threatened with imminent death or serious injury.
I read these posts of someone "knowing" what they are talking about, when they are repeating miss-information. Realize the difference between the two types of shooting situations. An intended shooting and an accidental discharge of a gun. Give it thought and you can see the difference of how modifications can be an issue, only if the gun discharged by accident.
Bob