The 1A show on NPR on May Day 2018 was on "Big Guns," a title of a book by former U.S. Representative in Congress Steve Israel, with the subtitle for the 1A broadcast, "Fighting Firearms With Funny." There was a significant rhetorical question raised, one I'd like to answer. The question was something like, "Who'd want cop-killer bullets?"
One answer to the question is, criminals sloppy enough to have to figure on getting into shoot-outs with police or pathological enough to really desire a shoot-out with police.
More significant, though, are those for whom the Second Amendment primarily protects "The Right of Revolution" and (mis)understand revolution as mostly partisan warfare against the military forces of the State. And who are the front line of those forces of the State? The police, who sometimes where protective gear that will resist passage of ordinary bullets.
This reading of "The Right of Revolution" also explains why one would want not just military-style weapons but military-grade weapons, 'cause that's what the military and paramilitary forces of the State have. The position usually involves paranoid fantasies of vast conspiracies by ZOG (the Zionist Occupied Government) or more fashionable embodiments of The World-Wide Conspiracy — and/or anticipations of race war — but after granting the assumptions, there is a logic to it. It's just not a logic you'll like if, say, you're a police officer or have police in the parts of the family you like or depend on the police for protection or dislike cops but not enough to want to kill them or want to improve the current American Republic and not overthrow it or have ideas about revolutions that are based more in history than in video games.
In the US, we tend to talk too much about "senseless violence"; here and elsewhere, we need to analyze the ways in which most acts of violence do make sense — but are unjustified and evil.