NL5

Throwing it aside, he wrenched the assault rifle from the nearest soldier out his hands and aimed it at the girl, squeezing down to nothing but a click and a lack of any gunfire. Shaking, he drew out the magazine, looking down at all the cartridges sat happily in their wells. One slid from the end of the gun's barrel. The back of the bullet had shattered, but it hadn't ignited. Dust dribbled out onto the floor.

"It nullifies dust!" the still-conscious egghead whispered in awe. "Not aura and semblance, but dust. All dust! Fuel, energy, ammunition, e–everything. Incredible. This is incredible, perhaps even more amazing than Null itself."

—NF5

[...]

I think all appliances and stuff in RWBY runs on dust. It's their primary energy source from what I remember, replacing oil, gunpowder and other things of that nature. Hence why the bullet wouldn't fire. The primer relies on dust, while the "propellant" would also presumably be dust.

I could be wrong on that, but it's how I figure it comparing bullets in the real world and dust. While we could just assume Remnant has gunpowder as well, I don't know if they do. It's always kind of implied dust handles everything there.

—NF5 Author's Note