That’s not the right reading of the convention. The rule is against unnecessary suffering and even excessive lethality. For example hollow point rounds are more effective at killing but solid rounds have the same disabling effect and give the recipient a chance of survival. Hollow point bullets are banned by the convention.

Its also illegal to use excessive calibre bullets against human targets. Even though such large projectiles are much more likely to kill.

The exact phrasing is:

“It is prohibited to employ weapons, projectiles and material and methods of warfare of a nature to cause superfluous injury or unnecessary suffering.”

https://ihl-databases.icrc.org/customary-ihl/eng/docs/v2_rul...

A sizable subset of America's past, present, and future adversaries in war have never ratified any portion the Geneva Convention.