>Full metal jackets are used for target practice
Combat. Their use is attributed to the Hague Convention of 1899 as a response to the prohibition on expanding projectiles in Article IV.
Declaration concerning the Prohibition of the Use of Bullets which can Easily Expand or Change their Form inside the Human Body such as Bullets with a Hard Covering which does not Completely Cover the Core, or containing Indentations.
This declaration states that, in any war between signatory powers, the parties will abstain from using "bullets which expand or flatten easily in the human body." This directly banned soft-point bullets (which had a partial metal jacket and an exposed tip) and "cross-tipped" bullets (which had a cross-shaped incision in their tip to aid in expansion, nicknamed "Dum Dums" from the Dum Dum Arsenal in India). It was ratified by all major powers, except the United States.
https://en.wikipedia.org/wiki/Hague_Conventions_of_1899_and_1907