Air Rifles--Literally.

[TOSDOC]

Having an unlimited supply implied no attached compressed tank, so I was imagining the hiss as an intake prior to firing. At 900/minute that sounds like a lot of hissing and popping.

[Formless]

What makes you think the hiss came prior to firing?

"Alex shouldered the rifle and then proceeded to aim at the tail section of the crashed plane and pull the trigger [my emphasis]. There was a hiss and a sound like air being blown through a pipe and a small section of the tail fin of the broken plane magically wanged off into the darkness."

That sounds to me like the author is saying that the hiss is the sound of it firing, when the narrator was (reasonably) expecting a loud bang. In fact, the description of the hiss as being like air "through a pipe" seems to be invoking the shape of the weapon's barrel for the audience.

[TOSDOC]

That's a good point, I was just trying to understand where the compression fits into the firing cycle. An unlimited ammunition supply implies the weapon is drawing from ambient air. This gives you a couple of possibilities: an intake during each cycle (like the cardboard box above), or a period after a given number of shots for a compression chamber to recharge (the larger cannon in the latter video). The former sounds favorable for a combat situation.

[Formless]

Well, the cannons in the latter two videos didn't use compressed air at all-- they used combustion chambers. In the one where it knocks down a wall the chamber merely mixes acetylene gas with air until it meets an optimal fuel to air mixture. The one that was testing knockdown power against a human target used gunpowder.

Neither of those designs would have "unlimited ammo". They would be limited by the amount of fuel you can carry. In fact, a modern rifle cartridge is mostly gunpowder by volume, not lead. Even though this rifle could theoretically grab air from the surrounding environment to create its projectile, it still needs an energy source just like any other machine in existence. This seems to be a misunderstanding by the author about what it means to reload.

[Terralthra]

My impression was simply that the author was mistaken about whether or not it needed gunpowder. My envisioning of the system was that it used essentially "blank" 5.56x45mm cartridges, and when you pulled the trigger, the action of the gun cycled as normal using gas exhaust, while the portion of the exhaust that would normally fire the bullet instead fired through a miniaturized vortex generator to compress and fire air.

It sounds similar to the Alliance carbines you see their troopers using in "Ariel" from Firefly in appearance. If I recall correctly, they too fired a semi-visible ball of air.