On this side of the pond Tort which is a law appertaining to civil wrongs and the catch all meaning of it is you cannot plead ignorance of the law. So if you adjust tamper or in any way fundamentally alter a firearm the law compels you to have it retested by the official body which is in our case the proof house failure to do this puts you in breach of our criminal law and Tort if you sold it on after you made the adjustments. Therefore you have no excuses when it comes to whatever action is taken against you be it criminal civil or both because you cannot plea ignorance of the Law.
Now if there are no Statutes appertaining to Gun Proofing on your side of the pond and a person tampers with a gun to such an extent that it is positively dangerous how can you take legal action against such a person because you would now have to prove that he intentionally made the firearm unsafe.
As shotgunjones pointed out, with very few exceptions we are free to modify our firearms as we wish. The exceptions typically have to do with alterations that might change a weapon's regulatory status as a so-called "assault rifle", machine gun, short-barreled rifle, sawed-off shotgun, etc. There is no law whatsoever (that I am aware of) that concerns itself with the
safety of alterations or modifications of firearms.
Concerning torts (negligence), I am not sure a person who alters a firearm necessarily owes a duty of care to a person who later uses that firearm. It would depend on the situation and circumstances. In any case the plaintiff must have actually suffered a loss or injury and be able to prove that it was caused by a failure (of the defendant) to exercise reasonable care. Assumption of risk would likely be a defense, as second-hand firearms are typically sold caveat emptor. Common sales terms are "sold as-is" and "it is the responsibility of the buyer to have it checked by a competent gunsmith."
