re: dumb youtube couple
FYI, "50cal" sounds impressive BUT, not all 50cal's are created equal. The Desert Eagle that's chambered in 50cal uses the .50 Action Express cartridge, which is 12.7x**33**. There's a BIG difference between that and the 12.7x99 (.50BMG) or 12.7x108 rounds, which is the typical round you'd think of when hearing "50 cal". A quick look at wikipedia shows that .50 BMG has ~18,000 J of energy; .50 AE has ~2,000. Still, that's a very powerful handgun round, probably not the most powerful (.500 S&W perhaps?) but greater than your typical 9x19.
As for tanks, you're not going to be damaging the vital parts of modern tanks with any KE threat short of an actual tank round; though anti-tank rifles did exist back in the day, but they would be 14.5 or 20mm. Huge fucking rifles. Generally speaking rifles (like barret 50cal) chambered in .50 BMG are used in anti-materiel role, so for taking out light vehicles, equipment... You can see videos of it on youtube against car engines.
But yeah, completely idiotic and irresponsible to fire a gun at another person, and doubly so without testing it first. It's incomprehensibly stupid. Never point your [loaded] gun at something you are not willing to destroy. An article I read claimed the boyfriend convinced her it was safe by showing another book that had been shot, but it wasn't mentioned if it was the same round, or the same type of book. Regardless... idiots.