[astine]

True, but the alternative is tungsten, which is not much better in terms of toxicity:

https://www.atsdr.cdc.gov/toxprofiles/tp186.pdf

The problem here is that armor piercing rounds need to be made of heavy metals in order to work and heavy metals are generally toxic. Even lead, which old fashioned bullets and shells are made of is toxic.

[regularfry]

Uranium is more likely to distribute that toxicity in a convenient dust cloud, though. Tungsten and lead for the most part stay in one piece on impact.