I don't know what the threshold is in other countries, but Canada has a vague but useful threshold: "knew or ought to have known". A different way it's been stated in the courts is "what a reasonably prudent person ought to have known"

Looking at the fuel line question through that lens: did the leak start while you were driving and you were completely unaware? Or has it been leaking for a while and you just haven't gotten around to fixing it.

With the ammunition, how was it stored? Was it dumped in a toolbox filled with pointy screws? (A reasonably prudent person ought to know that ammunition is fired by striking the primer with a sharp object). Was it stored in the front seat of the car on the hottest day of summer? (A reasonably prudent person would expect it to get really hot in there)

Etc. Etc. The thing about negligence is that there's a lot of room for interpretation. As another example, if you've been driving a car around with self-driving features, and you've experienced it behaving erratically multiple times, and that's followed by an accident... you knew or ought to have known that it was dangerous to be on the road in that vehicle. If there was an OTA update for the autopilot last night that installed silently, and it results in a crash, then it's probably the manufacturer who's liable.