I just rewatched The Winter Soldier the other day and a lot of the big guns on the helicarriers made me think about this. Does it make the bullet more accurate?
In: 6957
A normal machine gun has a single barrel and action that controls the insertion of a cartridge, its firing, and the ejection of its casing. This is fine for most things. But as you increase firerate, it runs into issues namely:
The cartridge case must be fully extracted before a new bullet can enter the chamber, so you’re fundamentally limited by the speed of the action, which is itself limited by material stress limits and recoil impulses the shooter can tolerate.
Heat builds up in the barrel (and action). Many machine guns if fired continuously can make the barrel glow red and even melt. Even before melting the harmonics of the barrel will change as it gets hot and it will trash your accuracy. Excessive heat can also prematurely detonate the propellant in the cartridge when it gets put in the chamber, which is *very bad*.
But what if we took a bunch of machine guns and put them together? We want every shot to have roughly the same trajectory, so we’ll make that easier by having them all fire from the same position. We can do this by making the whole assembly spin. Now we have 6 machine guns all in different parts of the load-fire-extract cycle, and we can get *much* higher firerates. How much higher?
The [M240 FN MAG](https://en.wikipedia.org/wiki/M240_machine_gun) and the [M134 Minigun](https://en.wikipedia.org/wiki/M134_Minigun) both fire the same 7.62x51mm NATO cartridge and are in use today.
The M240 can get between 650-950 rounds per minute (or ~11-16 bullets per second if you like that comparison better) depending on burst size – you can’t stay at the high end for very long else you’ll run into heat issues as discussed above.
The M134 can fire between 2000-6500 rounds per minute. That could be over 33-108 bullets every *second*. That’s a lot of firepower. And you can sustain it for longer since the heat buildup is managed for each barrel. Assuming you can supply the ammo, of course.
Now why are rotary cannons used on aircraft? Planes and to a lesser extent helicopters tend to be moving very fast. They might also be maneuvering to avoid fire. So they want to get as many bullets thrown at their target in the very short window they have to fire. So they opt for rotary cannons, usually with some form of explosive or incendiary round. They can’t sustain that fire for very long, most fighter jets now only carry a few hundred rounds at max if they still even have guns. You’ll also see them used on some anti-aircraft or anti missile systems such as the [Phalanx CIWS](https://en.wikipedia.org/wiki/Phalanx_CIWS).
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