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[Culise]

The method exists.
Basically a static shroud around the rotating barrels that has the liquid inside to draw away the heat.

Read through that whole thing and still barely understand
It looks like a simple fixed boom which sprays pressurised gas and coolant over the barrels as they spin. The coolant isn't a problem- just pack it with the ammo, I guess. Heck, it could actually make for a good countermeasure if the ship's magazine gets hit.
Not sure how efficient it would be but the "rolling heatsink" idea I had was probably more awful.
Plus it could compensate for the spin... Although I'm not sure about the kick of the gun.
I have no idea how guns kick in space.

Pretty much.  I doubt it would compensate for the spin, unless you angled the jets appropriately, but then you'd be applying your countervailing force directly to the spinning barrels.  That would seem rather inefficient to me, because it directly opposes the rotation that you need to fire.  It'd be better to, for example, introduce a countergyro behind the barrel inside the firing mechanism; you could even design it as part of the aiming mechanism to stabilize the weapon, just hypothesizing off the top of my head. 

As for how guns kick in space, it's exactly the same as on Earth - a directed, powerful, and short-term force directly opposite the trajectory of the bullet.  It's the impact on the firing body that makes this interesting; without water pressure (ships) or friction (person or land vehicle) to counteract this, you could easily end up spinning like a Popovka if you don't introduce some other of absorbing or counteracting this force.  Counter-thrusters and gyros are both options for active counterbalancing that come to mind.  Alternately, designing the mechanism to fire along a line intersecting the center of mass is a passive method to negate rotational moment introduced by recoil, and shock mechanisms can be used to cushion the force. 

Also, fun fact on rotary shotguns - apparently, they do exist.  Well, specifically, shotguns with rotary (revolver) magazines; I've never heard of a shotgun with a rotary barrel, which is what seems to be what everyone else is discussing.  It's just a tool for rapid reload. ^_^

[Grimlocke]

Hmmm, shotguns in space might not be the worst idea.

Velocity differences in vacuum get pretty big pretty pretty easily, to the point where chemically propelled projectiles would be going slower than the target, but being able to spread a big cloud of bits of shrapnel in front of something with a couple km/s speed difference should make an impact at least.

Though even then something like canister shot would probably still work better than anything that fires a stream of much smaller projectile clouds.

You could potentially use a rotary gun as an improvised thruster, with a slight risk of collateral damage.

[Kot]

I read this spess book (I think it was one of the Uplift series ones) where chased human (actually more like dolphin) ship ejected a cloud of water behind them while going at SANIC speeds in space, which caused said water to turn into ice crystals and the pursuing aliens had only a brief window of time in which they could only say something along the lines of "OH SHI-" before getting torn apart by the impact.
If we think of very fast ships, spraying fields of low velocity clouds of whatever around might not be that bad weapon.

[GiglameshDespair]

Also, fun fact on rotary shotguns - apparently, they do exist.  Well, specifically, shotguns with rotary (revolver) magazines; I've never heard of a shotgun with a rotary barrel, which is what seems to be what everyone else is discussing.  It's just a tool for rapid reload. ^_^

That would probably be a revolver shotgun, rather than a rotary shotgun. Small but distinct difference. Multi-barreled revolving small arms were often called pepperboxes. A 8-barreled pepperbox shotgun did exist, called the Colt Defender, but it was never adopted as a weapon by police or military.

[Culise]

Also, fun fact on rotary shotguns - apparently, they do exist.  Well, specifically, shotguns with rotary (revolver) magazines; I've never heard of a shotgun with a rotary barrel, which is what seems to be what everyone else is discussing.  It's just a tool for rapid reload. ^_^

That would probably be a revolver shotgun, rather than a rotary shotgun. Small but distinct difference. Multi-barreled revolving small arms were often called pepperboxes. A 8-barreled pepperbox shotgun did exist, called the Colt Defender, but it was never adopted as a weapon by police or military.

All I knows is that the people who make it call it a rotary-mag shotgun. But yes, I expect that's more of a marketing tool than anything. ^_^

[Tack]

So you'd need to compensate for kick, spin and coolant.
Ball-bearings in fluid could make an acceptable free-spinning heat exchanger, except things able to contain fluids are notoriously bad at handling friction, killing the fluid, and bearings would end up generating their own heat anyway.

... Gah. Now I've got to think of a way to make the barrels stop.

This is a surprisingly fun thought experiment for me. Somehow I keep coming back to it.

[Tuxfanturnip]

Speaking of shrapnel and the relative velocities thereof, there was some discussion of mines earlier... Why use mines when you can use rocks? Put a nice big asteroid in whatever orbital plane you want to render unusable and go at it with a jackhammer for a few weeks, scattering shards of rock into slightly different orbits that will soon form a ring. Better yet, do this in 3 orthogonal orbits, inducing Kessler Syndrome in a way that should eventually create a sphere of gravel in varying and unpredictable orbits equally covering the planet. Put it far enough out and it will take more material, but will mostly spare GPS satellites and space telescopes, the infrastructure needed to look out for incoming enemies.

Granted, thick enough armor will protect against this defense, but it could probably deter lighter craft, missiles, and civilian craft from the planet itself. It might be more useful as a siege tactic if constructed quickly enough, destroying the defenders' orbital infrastructure in one sweep and putting any new launches at risk.

With revolving weapons, maybe better materials science would allow the barrels themselves to have internal channels, with coolant pumped in through a central valve  and drained from an outer ring after zigzagging along the barrels. But then you'd need rotating gaskets... Maybe the rotation would help the coolant stay in the system even with an imperfect seal, if it "pools" every time it's transferred outward.

[Tack]

I had thought about a "centrifugal pump" system so that the coolant could "spray" across back into a catch-plate.
But the second half of that idea seems a little unfeasible.

[Parsely]

Speaking of shrapnel and the relative velocities thereof, there was some discussion of mines earlier... Why use mines when you can use rocks? Put a nice big asteroid in whatever orbital plane you want to render unusable and go at it with a jackhammer for a few weeks, scattering shards of rock into slightly different orbits that will soon form a ring. Better yet, do this in 3 orthogonal orbits, inducing Kessler Syndrome in a way that should eventually create a sphere of gravel in varying and unpredictable orbits equally covering the planet. Put it far enough out and it will take more material, but will mostly spare GPS satellites and space telescopes, the infrastructure needed to look out for incoming enemies.

Granted, thick enough armor will protect against this defense, but it could probably deter lighter craft, missiles, and civilian craft from the planet itself. It might be more useful as a siege tactic if constructed quickly enough, destroying the defenders' orbital infrastructure in one sweep and putting any new launches at risk.

This is only a good idea if you plan on ruining that apparently livable planet for a very long time.

Unless the ships have very powerful engines and very short ranges then I doubt any vessel would survive.

I read this spess book (I think it was one of the Uplift series ones) where chased human (actually more like dolphin) ship ejected a cloud of water behind them while going at SANIC speeds in space, which caused said water to turn into ice crystals and the pursuing aliens had only a brief window of time in which they could only say something along the lines of "OH SHI-" before getting torn apart by the impact.
If we think of very fast ships, spraying fields of low velocity clouds of whatever around might not be that bad weapon.

Did the humans slow down a lot first? Because passively dropped projectiles only work as a deterrent if the ship following you is going much, much faster than you, otherwise you would just be gently exfoliating their hull as they matched speed before drifting through the cloud.

[Tuxfanturnip]

Not exactly spray, but this design would minimize the contact between the coolant and the (red) rotating gaskets. The channel would split at the entrance to zigzag "down" both sides. Honestly, the best primary coolant for this system might be mercury, whose density would cause it to flow faster through the channels without pumping, and whose low vapor pressure might even eliminate the need for gaskets entirely, if some coolant loss is acceptable. You could also use the mercury for an independent droplet radiator, if you want a redundant, less vulnerable cooling system and aren't going to fire under acceleration too often.

This is only a good idea if you plan on ruining that apparently livable planet for a very long time.

Isn't this basically what modern minefields end up doing?

[Egan_BW]

I imagine it's the kind of thing that an isolationist civilization would do. Like building a space-wall.

[Kot]

Did the humans slow down a lot first? Because passively dropped projectiles only work as a deterrent if the ship following you is going much, much faster than you, otherwise you would just be gently exfoliating their hull as they matched speed before drifting through the cloud.

The ship was much faster because IIRC they were using gravity assist from nearby gas giant which also covered them up for long enough that the aliens following in their path didin't have time to notice a huge cloud of stationary ice and human ship running away in opposite direction... or something.

[Parsely]

This is only a good idea if you plan on ruining that apparently livable planet for a very long time.

Isn't this basically what modern minefields end up doing?

Permanently blocking a planet from space access is more like permanently closing a country's borders.

The ship was much faster because IIRC they were using gravity assist from nearby gas giant which also covered them up for long enough that the aliens following in their path didin't have time to notice a huge cloud of stationary ice and human ship running away in opposite direction... or something.

The ice wouldn't be "stationary" unless the human ship came to a complete stop first. If they dropped the fluid from their ship during their slingshot maneuver it (the ice) would be moving at the same speed as the ship. If the ice was stationary relative to the alien ship then it would have to have been fired (as opposed to released) from the human ship towards the alien ship, at exactly the speed that the alien ship was approaching.

[Egan_BW]

The water would probaby boil, not freeze.

[Kot]

The ice wouldn't be "stationary" unless the human ship came to a complete stop first. If they dropped the fluid from their ship during their slingshot maneuver it (the ice) would be moving at the same speed as the ship. If the ice was stationary relative to the alien ship then it would have to have been fired (as opposed to released) from the human ship towards the alien ship, at exactly the speed that the alien ship was approaching.

running away in opposite direction

They did (well, it wasn't proably complete stop but enough to make a difference). It was only logical for the aliens that the humans ship would try to gain as much speed as they could from the gravity assist and run away but instead the humans (actually again, more like dolphin) faked it, pulled high-g maneuver, used the gas giant cover to stop and relase the water and then go in some other direction.

EDIT:
NOTE THAT I HAVE NO ACTUAL IDEA BUT THIS IS HOW I REMEMBER IT. The book was David Brin's Startide Rising, by the way.

The water would probaby boil, not freeze.

Actually, both, IIRC. First it freezes then it boils and then it freezes or someshit. And even if it was just gas, at high speeds it wouldn't be very amazing to fly into it.