Yup. Railguns have a heating problem currently too, admittedly, but coilguns could actually benefit from firing in a vacuum. My main point here is that electromagnetic acceleration is vastly superior to today’s weapons: extremely high acceleration of the slug within centimeters, no moving mechanical parts, and reliance only on ammunition and electricity (which would be readily available on a spaceship, although the amount of power draw is significant (which could be used as a reason for diverting power to the various systems to increase rate of fire)).
However the initial point is that weapons used in space will rely on mass-less propulsion wherever possible, thus precluding the use of gunpowder (or any other combustion material). I don’t think anyone would argue against this? I’d be interested to hear a case for it however in case I’m missing some facts (would gunpowder even fire in a vacuum, or would the firing chamber need to be “inside” the pressure vessel?).
On the use of Kinetic Weapons
To make my case against (admittedly cool) rotary canons I’ll start by looking at why we’d use a kinetic energy delivery system as a weapon in space and why railguns / coilguns are better suited to it. I’ll then briefly highlight why I think rotary canons (even as railguns or coilguns) make little sense in my opinion.
Kinetic weapons work by accelerating a mass to high speed and pointing it at someone we have a disagreement with. On impact Newton makes sure that a whole lot of energy is transferred. There are two parameters that I’ll look at that are mainly important to make a kinetic weapon deadly: speed of the projectile and the mass of the same. The goal is to have as heavy a projectile impacting at a as high of a speed as possible. The higher the speed the better the chance we actually hit something: modern weapons are good for a few kilometers; but combat in space will start at thousands of kilometers (beyond kinetic weapons we need self guiding weapons, so torpedoes / missiles). The faster the projectile the less time for the target to evade.
And here we have the first arguments for magnetic acceleration: railguns / coilguns have a theoretical unlimited acceleration, limited chiefly by the amount of power you can put into it at a moments notice. Additionally the acceleration is created by the gun itself, removing the need for the cartridge. This in turn allows for more mass of the projectile to be used as simple mass (small footnote: I believe that future slugs would additionally be self steering, allowing continuous correction while in flight to increase effective range).
All right, so magnetic acceleration is the solution. But why are rotary canons unnecessary?
Rotary canons are a solution we use to increase the RPM of conventional weapons. But coilguns have non of the problems that rotary canons try to solve.
Heating, while still an issue, is not as much an issue for magnetic acceleration because the slug a) doesn’t even touch the barrel in the case of coilguns and b) the efficiency of electromagnetic acceleration is a lot higher than by explosive force, resulting in comparatively little waste heat. There still is waste heat, of course, but it should be way more manageable.
Rapid automatic reloading is a non issue for magnetic acceleration: the slug must simple be fed into the back of the acceleration structure. This can be as simple as pushing it into the barrel continuously. Since the recoil doesn’t need to be transferred from the cartridge to the gun, the cap of the barrel in the back that conventional weapons require is unneeded. The acceleration structure is thus a simple open ended cylinder, where slugs are pushed into the barrel from one side and exit it very quickly on the other side. And since we need no time to physically reload a railgun / coilgun (note that energy must be available, but that does not hinder the speed at which slugs could be loaded into the barrel) we can theoretically shoot as many slugs from a single barrel as physically fit inside of it at the same time, safe for the spacing we require to account for the acceleration of following slugs.
And that is, I think, the actual RPM limit that railguns / coilguns will be constrained by in the future. But this issue is solved a lot simpler by having multiple barrels next to each other without them rotating.
To top this off the problem with a kinetic weapon that has a high RPM fire rate will in the end always be determined by something else: the capacity of the slugs you can carry with you to shoot with. Once you have a solid firing solution you want to accelerate a large amount of projectiles at it in as short of a time as you can – but you’ll never hold down the trigger and empty your magazine at once because then you’ll have nothing for the next target. Realistically the gun would only fire a short burst, containing enough slugs travelling at a sufficiently high speed that they’ll hit with enough kinetic force to punch a hole in the other parties spaceship.
Rotating coilguns / railguns are completely unnecessary from a realism point of view, adding unnecessary mechanical complexity for no gain.