I'm guessing they'd want a sample to spend a few thousand dollars on testing before saying it was a firearm.
Snarkiness aside, I would side on the idea that since it's an explosive propelling a projectile it would be considered a firearm.
On the more complex side, could a powder charge be ignited with the cap as a primer or is the ball fired by gases that are not hot enough to ignite powder? That would be my suspicion if they would not be considered firearms. Paintball guns are sold without a 4473 and they use pressurized gas to propel a projectile. Some air-soft guns do as well.
Disequilibrium facilitates accommodation.
9mm vs .45 ACP? The answer is .429