system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 What is full metal jacket ammo used for? How does it differ from normal ammo

How does it differ from normal ammo and in what ways is it better?
gpt
Full metal jacket ammunition has some mechanical benefits, but the real reason that full metal jacket bullets are still used is the 1899 Hague Convention.

For most of the 1800s bullets were all soft lead. However toward the mid-late 1800s better propellant was developed, which caused more lead to be deposited in the barrel with traditional bullets. Full metal jacket bullets were developed to address that problem by surrounding soft lead with a metal that was less likely to foil the barrel at high velocities. 

Problem was, the terminal effect of full metal jacket bullets (ie the injuries they caused) were generally less significant than a soft lead bullet. So, the British in India, at the Dum Dum Arsenal (hence dum dum bullet) developed a bullet that had a partial jacket but a soft lead deformable tip. In theory, less barrel fouling, same wounding capacity as a traditional soft lead bullet. 

The Germans decided to start a diplomatic kerfuffle over the use of the bullets, and that resulted in Article III of The Hague Convention which prevented the use of these “expanding” bullets in *war.*. So most militaries use full metal jacket ammunition as a means of complying with international law since FMV bullets aren’t designed to expand.