The Boran X Ammo is ammunition for the Boran X in The Angel of Darkness. It's first found in the level The Sanitarium.
Since the Boran X Ammo isn't a very common pickup, it's not worth wasting it, escpecially when you need it the most while fighting the Proto and Boaz.
"High Impact 9mm Ammo" 
- ↑ In-game inventory