The item's main idea of the plane propeller and machine gun, are designed after the concept of a "synchronization gear." Though sometimes is is also called an "Interrupter Gear"

What's a "Synchronization Gear," i hear you ask?

"A synchronization gear, or a gun synchronizer, sometimes rather less accurately called an interrupter, is attached to the armament of a single-engine tractor-configuration aircraft so it can fire through the arc of its spinning propeller without bullets striking the blades."

Since aiming a gun independently from flying your plane was rather difficult, one man got the idea to strap his gun in front of the cockpit so that he could fly an aim at the same time. That man's idea of damage prevention was simply metal plates on the propeller, while it worked and he was the first man to successful to down an aircraft with this manner of attack, somebody else came up with a better, less risky way to go about it.

This video can probably explain it better than i can, so give it a watch:

https://www.youtube.com/watch?v=VX3fDlR-SDw

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Obviously having to work with the stock animations that Valve has, the gun could not be mounted on top of the engine like i would have preferred, as the rotating propellors needed to stay aligned with the minigun's spinning bones. The idea remains mostly the same however, here's an old doodle from when i first came up with the idea a few years back:

https://puu.sh/BhEkM.jpg

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"But wait, why not just make a regular gun instead of this contraption? The heavy isn't flying a plane so it makes no sense!"

A good argument, yes, but we just wanted to do something silly. Not everything needs a good reason behind it.

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Regardless, we will also be making a proper Lewis gun for the heavy in the future, which the under-mounted gun is partially based off of.

It may be a while before we get to it, but if you think this is a tad too silly for your tastes, hopefully the other one will be more to your liking whenever we get around to it.