In the following steps:

1. It's often cheaper to load real guns with blanks than to buy prop guns. Blanks are very dangerous at short range because they have a lot of energy.
2. The prop master and the firearm safety expert fucked up enormously
3. Baldwin mishandled the gun and fired it.

So uh, capitalism. Cheaper to use a real gun with pretendsies ammo (that can still kill you) than a fake pop-pop gun.