Theoretically, the particles themselves could be drawn from the air, if the technology was advanced enough; particle beams developed in the '80s used hydrogen.
You didn't actually read the Gyrojet article, did you? :wink: Rifling the barrel would cause severe problems, which is why the Gyrojet used vents for stabilization rather than rifling the barrel. Rifling fins could also work, though as I mentioned earlier, the bolt in the Ultramarines movie appears to have some sort of rotary stabilizer band.Then after it leaves the barrel it ignites a secondary rocket propellant that actually carries it to its target, using the technology known today as Gyrojet (Wiki link here), which can use shaped nozzles to add more spin to the round without requiring fins or stabilizers.
The only thing the rifling does is get it started so that the jets aren't starting from a situation of no rotation at all. This means the barrel doesn't have to be as long as it would for a standard bullet.