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?
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There are two benefits to a minigun-style barrel.
First off, the barrel is a failure point for a weapon. It gets hot from bullets firing. If you try and shoot too many bullets through it too quickly, it can overheat and fail pretty catastrophically. By rotating barrels, you are only shooting a fraction of the bullets through any barrel, giving them a bit of time to cool off between each bullet, increasing the time before it has to stop firing due to overheating.
In addition, with most guns, you need to load a bullet, fire it, and then unload the casing before it can fire again. With a minigun, you can accomplish these tasks in parallel — you can be loading a bullet in barrel 1 while firing one in barrel 2 and unloading one in barrel 3. This can speed up your effective fire rate quite a bit.
It’s for two main reasons
1. Rate of fire – weapon’s time to fire one bullet is defined as “function cycle”=all actions of the gun from firing a bullet, trough cycling the weapon, to making it ready to fire again. your average assault rifle can only have one function cycle running at a time. By having multiple barrels, you can effectively having multiple function cycles running at the same time, all of them going off in the exact same spot, which makes the gun fire extremly fast (4000-6000 rounds per minute, faster firing machine guns have around 600-800 rounds per minute).
2. Cooling, as others already pointed out, by having multiple barrels rotating, this increases the airflow so that the guns don’t overheat “as fast”, that doesn’t mean it can’t melt with firing too many rounds
Some other comments have explained that the rotating barrel helps with shooting speed by keeping the barrels cooler and by reducing time between shots, but they haven’t said why they put these types of guns on aircraft.
The answer is that it is really, really hard to hit a target that is moving fast, or to hit a slow moving target when you are moving fast. If you want to increase the odds of a hit, you need to fire a lot of bullets as quickly as possible, so that the chances of at least one of them hitting the target is reasonable.
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).
Other posters have addressed why they’re used (cooling/rapid fire), but no one addressed “does it make the bullet more accurate?”
Yes, it does.
As a barrel heats up, it expands, which means the projectile doesn’t fit as snugly, so it’s not as accurate. By keeping barrels cool, accuracy is increased.
One of the most famous examples is the A-10 Warthog, which they discovered during testing was actually *too accurate.* The bullets all hit in a straight line, even after traveling thousands of feet. They added an offset weight to the end of the barrel ([the little round thing above the central nut](https://nationalinterest.org/sites/default/files/main_images/Cannon_0.jpg)), which made the whole assembly wiggle and decreased accuracy.
Most of these answers are missing the point.
In an aircraft you are moving rather quickly and don’t have a lot of time to shoot on target, so you need a lot of fire down range in split second while your crosshairs are on target.
It’s because of this that they have rotating barrels etc…. To manage heat and increase fire rate.
Also, why is it “mini” gun when it is, in fact, a pretty big gun?
Former A10 maintainer here, and while I was not munitions myself several friends were and I am intimately familiar with the GAU-8 30mm rotary barrel autocannon system from helping them service the guns. First is barrel heat management. The more rounds you fire through a barrel in succession, the more heat is built up in the barrel. Managing this heat build up is a major limiting factor for fire rate from the gun, as if you build up too much heat, the barrel loses integrity and can either begin to expand from the pressure of the propellant powder combustion or begin to melt and sag, both of which will lead to very catastrophic failure. This heat build up can be managed with good gas recirculation design, but you can only fire so many rounds through a single barrel at a limited rate of fire before the barrel fails. With a rotary barrel, you’re firing a round through the first barrel, then it moves out of the way to fire the next round through the next barrel. So if you have a 7 barrel weapon like the 30mm, you’re giving the first barrel 700% longer to cool to achieve the same rate of fire. Also, barrel cooling is greatly improved from the barrels spinning. Single barrel weapons can only fire continuously in the 500-800 round per minute range, and can be upcycled to around 1000 with increased barrel wear. Most rotary weapons fire in at least the 2500 rpm range and many can be over 4000 rpm continuously. Also, you can divide tasks amongst the separate barrels. You can load each barrel in one station, fire it at the next, and extract the spent casing at the next. So instead of having to wait to do all three before moving to the next round, all three can be running at the same time in separate barrels. But the primary reason for multiple rotating barrels is barrel heat management for increased rate of fire.
Because it is cooler (literally cause overheating). Rotating barrels allow for rapid fire by performing simultaneous tasks (load, aim, fire, discard), they also reduce recoil which improves targeting stability. There is of course a limit, as you cannot really have a rotor-barreled canon (the big gun on ships). Technically, it’s possible, but practically, you’d have to be fighting like a billion ships. The mini-guns work because rapid fire is good for relatively smaller targets.
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