Robbery Suspect Tracked by GPS and Killed

Actually, the smokeless powder in handgun ammo pretty much doesn't finish burning up by the time the bullet exits, resulting in a muzzle flash problem at nighttime, and that burning powder ought to do a good job of burning up any chemical taggant. It's probably telling the gun grabbers have never seriously suggested this (vs. tagging powder or brass).

Then there's the traceability problem. Lots of ammo is bought in bulk, especially by police departments, it would cost vastly more to get customized ammo for each officer, be a total nightmare to try to get it to designated officers and keep it isolated to them. You can't realistically demand an officer never hand a spare magazine to another, especially in a firefight, and ammo is generally a fungible item.

It would never work in a civilian context, it would be vastly more expensive as noted above, we'd boycott any manufacturer who tried such a stunt, and since in the US we buy the vast majority of ammo it would stick.

Also note US civilian ammo production for all types is above 12 billion rounds per year, and I've read rimfire ammo (e.g. .22 LR, not considered by almost everyone as suitable for self-defense) is only 3-4 billion of that. Those numbers strongly suggest taggant approaches are impractical.