It's used in the firearms community instead of anything with the word "accident" because there's almost never a reason for a gun to fire unintentionally that doesn't involve a serious fuckup by the user.
Any ND that results in death or injury should also be treated as a felony. There are 4 basic rules to firearm safety that should be followed at all times. Any one of them should prevent injury, so you have to fuck up all 4 at once to hurt someone.
Treat every gun as if it were loaded at all times.
Keep your finger away from the trigger until ready to fire.
Do not let the gun point at anything you're not willing to kill or destroy
Know your target and what's beyond your target.
The only true accidents I know of regarding a gun being fired unintentionally involve faulty weapons firing on their own (more likely to win the lottery), but even then they shouldn't hurt anyone because it should be pointed in a safe direction.