You're a bit confused on the terminology. "Dum dum" is a old colloquialism for an expanding bullet. Hollow points are a variety of expanding bullet, and the description from the Hague convention article III describes, and is intended to describe, expanding bullets. If it helps, you can look up some photos. For example, look up a photo of a Federal 115gr JHP bullet, one of the most widely used in law enforcement. That big hole in the front ain't for accuracy; it causes the bullet to expand like a mushroom after impact.
Interestingly, just last year the US Army approved the use of expanding bullets in M17 and M18 service pistols (both Sig P320). The issued ammo is Winchester XM1153, and yes, the Army (and everyone else) refers to this as a hollow point round.
EDIT: Also note that the Army disagrees with you; their justification for approving the use of hollow points is not that they're allowed under the Hague Convention, but that U.S. is not bound by it in many cases, such as warfare against irregular fighters, which describes pretty much all military conflicts for the last decade.
I specifically referred to 5.56 OTM, which is designed to operate exactly as I said, not 9mm hollow points, -- although I carried both 5.56 OTM and 9mmJHP in the mid-2000s in Afghanistan in the US military, so the Army's approval is not particularly relevant.
Also "dum dum"is a type of bullet (and I'm old enough to remember) that described a soft lead bullet that often did expand, not a hollow point. Hollow points were not in general use back then though they did exist.
I think I see the source of the confusion. The open-tip ammo you carried in Afghanistan (MK 262, or Black Hills 77gr for us civies) is not hollow point. Nobody calls it hollow point; not the Army, not SOCOM, not match shooters, nobody. So yes, I agree OTM is not covered by Hague Art. III.
But I seem to remember you saying that Hague Art. III doesn't cover HP ammo at all, which is incorrect. If you carried HP ammo in Afghanistan, it was allowed (strictly speaking) because Hague doesn't apply to conflict against irregular fighters. The U.S. military openly acknowledges that Hague prohibits HP ammo in regular warfare between signatories, and that information is everywhere you look if you follow any news or debates about military firearms. For example, the big news about the new M17 isn't the gun, but the fact that HP pistol ammo is now standard-issue, and how the Army addresses Hague Art. III in justifying the change [1].
The US Military, in my experience, has never actually applied the Hague convention to hollow points. There may be plenty of legal articles, but practice has been different. I know for a fact socom has been using them since the 80s and maybe before.
I agree the whole thing is stupid. Every law enforcement officer in the country carries hollow point ammo. I think the military will keep nominally requiring ball ammo the next time we fight a conventional war, but not be too concerned if soldiers mix in some HP now and then. As you point out, not too different than what they already do.
Interestingly, just last year the US Army approved the use of expanding bullets in M17 and M18 service pistols (both Sig P320). The issued ammo is Winchester XM1153, and yes, the Army (and everyone else) refers to this as a hollow point round.
EDIT: Also note that the Army disagrees with you; their justification for approving the use of hollow points is not that they're allowed under the Hague Convention, but that U.S. is not bound by it in many cases, such as warfare against irregular fighters, which describes pretty much all military conflicts for the last decade.