A short checkup, seems to reveal that we are wrongly attributing the ban on HP ammo to the Geneva convention, while more likely it is from the Hague convention.
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Legality
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.40 S&W round, complete cartridge and expanded bulletThe
Hague Convention of 1899, Declaration III, prohibited the use in
international warfare of bullets that easily expand or flatten in the body.
[3] This is often incorrectly believed to be prohibited in the
Geneva Conventions, but it significantly predates those conventions, and is in fact a continuance of the
St. Petersburg Declaration of 1868, which banned exploding projectiles of less than 400 grams, as well as weapons designed to aggravate injured soldiers or make their death inevitable.
NATO members do not use small arms ammunition that is prohibited by the Hague Convention.
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
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Evidently, by the words of the first sentence above...hollow points per se, are not prohibited, but rather any bullet which expands or flattens easily. Of course, with today's long ranges and body armor..plus the tumbling effect..pretty well negates the old "flattening bullet" rule..