A colleague asks: "[I'm] wondering if hollow point bullets are legal and if so, what is the rationale given by gun fanciers for that rule? I know under the international law of war they are not legal in combat, but I take it the same isn’t clear in domestic law."
Many of our readers doubtless know more details than I do, but I think I can summarize the answer fairly well.
The bullets are indeed legal for civilian and police use. Wikipedia puts well the two reasons why many people prefer them, "Despite the ban on military use, hollow point bullets are one of the most common types of civilian and police ammunition, due largely to the reduced risk of bystanders being hit by over-penetrating or ricocheted bullets, and the increased speed of incapacitation."
To unpack this: (1) Because hollow-points deform on impact, they're unlikely to go through or off walls (or through the target and into the person behind him).
(2) If you're trying to defend yourself against attack, your goal isn't just to hit the person, but knock him down. Even a fatal wound might leave the attacker mobile enough long enough for him to kill you (either with his own gun, or with some other weapon if he doesn't have a gun). A hit with a hollow-point is much likelier to knock him down. It is also likelier to kill him, but that's a side effect, not the goal; the goal is for him to stop going at you.
Here's a 1998 story about the NYPD's adopting hollow-points (though I'm not sure whether the NYPD still uses them). The Police Commissioner is quoted as saying,
We are, in fact, going to switch to hollow-point ammunition as soon as we receive it. They are much safer than fully jacketed bullets, which will go through a person or tumble through a person's organs and then continue on and hit innocent victims.... It is the standard around the world in law enforcement to use hollow points.
The story also notes that "Other police officials have pushed for the bullets because they are more effective in stopping dangerous criminals, and they say that aspect further protects bystanders because officers have to fire fewer shots to incapacitate their targets." Likewise, "'[The hollow-point bullet] increases the wound's capacity to the victim, but it reduces a risk that the police are always concerned about: the risk of the bullet perforating the intended target and injuring a bystander,' said Dr. Stephen Hargarten, the director of the Firearm Injury Center in Milwaukee, Wis."