Actually, both the CDC and FBI use 500,000 as a baseline figure.
There are some guns that actually can “go off” such as the M1911 and M1911A1 .45 ACP pistol and similar designs that use a floating firing pin. Colt pistols have had a firing pin safety since the Series 80 was introduced in the early 1990s. But accidents involving these types of handgun are extremely rare because it takes a certain kind of drop to cause the firing pin to move forward where it can strike the cartridge primer.
Most long gun safeties operate either on the trigger or the sear.
The situation Mr. Thompson described was not an accidental discharge. It was negligent manslaughter and a clear case of a person who should not have been able to have a firearm having one anyway. They were criminals and engaged in criminal activities. This is not only federal law, it is state law in every state in the Union.
Your comment about roads is irrelevant because the majority of crimes committed by people in a state are committed by residents of that state so multi-state issues aren’t even a factor.