If the contention is that the defendant is negligent in his modifying the trigger. You have to prove that negligence.
In many cases, defendants have decided to settle out of court rather than risk a jury-determined award.
If a plaintiff
does have to prove the case, the standard of proof is a
preponderance of the evidence--that it at least
somewhat more likely that the trigger pull modification contributed to the loss than that it did not.
Talking pull weight. . . .You can get opinions that x.xxlb triggers are unsafe for carry. Even some "expert". Still, there is no data or fact that can prove this unsafe.
There are
all kinds of data that can persuade the triers of fact that light trigger pulls may have contributed to unintentional shootings. It happens.
Any "expert" discussing pull weight is sharing opinion based on their narrow view of the general public.
Along with verifiable data concerning the physiological effects of stress on fine motor skills.
Equally importantly, commercial firearms that are not intended for competition are equipped with triggers that have at least a certain pull weight for the particular model.
When one departs from the standard by using, for defensive carry purposes, a trigger with lighter pull than that with which the gun was originally sold, one puts himself or herself at risk. Competition is one thing; defensive carry is another.
So you would have to produce an "expert" to the contrary...
Good luck with that.
...or just make theirs look like an idiot.
How would one go about that?
It is likely that an expert for the prosecution or for the plaintiff is
extremely adept at testifying and at handling cross examination, and is
extremely knowledgeable when it comes to issues involving the design and use of firearms.
An expert for the defense, if one could find one willing to testify, would be hard-pressed to persuade the triers of fact that a modification that resulted in a lighter pull weight did
not potentially contribute to the risk of an unintentional discharge.
The defendant has on their side that there are millions of dollars spent annually on the triggers of working guns.
How would that help the defendant?
If what one wants is a
smoother trigger, there is likely no material risk there. If one wants to lighten the pull weight for competition, fine--just be sure to keep the gun pointed down range. But if the pull weight of a defensive firearm is heavier than the shooter prefers,
either make sure that the modification involves replacement by another stock trigger intended for defensive carry,
or choose another firearm.
After the shooting, the gun is available for your paid expert to review the gun and complete a function check. That is how you will prove your gun safe. Your expert will test it. You could pay ahead of time to have it inspected and that would be good. Not sure what kind of testimony you will get out of that person or if it is reasonable and customary to expect you do this. Passing a properly executed function check will show you pulled the trigger.
None of that would be helpful.
That the gun goes off when the trigger is pressed is not something that the defense would need to prove.
Let's review the bidding:
In a civil trial, the issue is two-fold: (1) whether the shooter, acting under stress, may have fired a shot that he or she did not intend to fire; and (2) whether it is likely that the use of a trigger with a lighter pull weight than that with which the manufacturer had equipped the firearm contributed to the event.
In a criminal negligence trial the issues are the same, but the burden of proof is much higher.
There is another potential issue in a criminal trial: evidence that the shooter lightened the trigger pull weight can be taken as evidence of a predisposition to violence.
I had a carry firearm modified some time ago. Two things were done: the trigger was smoothed, but not lightened; and the hammer on the revolver was bobbed, both to reduce the risk of snags and to eliminate any possibility of an argument that I had used the gun in single action mode and had fired unintentionally.
As Loosedhorse points out all of the risks we have been discussing here are avoidable.