I’d think the Army would agree to buy for the USAF as it would be dirty nasty beautiful Soldiers underneath the Hercs- kinda like a “vested” interest in decent ammo.
On the other hand, the Army brass should look at the USAF budget and tell them to cut out about half a billion in fluffy garbage that does not bring steel to the enemy....and buy their own damned bullets.
Something happened to bring select, not all, rounds under the Army for acquisition. Odd. The USAF wants to buy in accordance to their requirements, and I think this buy is driven by SPecOps, and SpecOps is under a different line item.
The Navy is (was?) the primary procurement agency for A/A missiles. Why? I have no idea but maybe if block-buys reduce cost. Who knows.