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I'm putting together a design for a large land based walker style weapon, the machine has missile rack above and 2 large railguns/coilguns/cannons on each side. Aesthetically I'd like the design of the cannons to have between 3-6 barrels and law wise be a coilgun. The weapons are intended for anti-infantry and combating large horde-like enemies at ranges of 500m to 4000m, I'm thinking around 300 rounds/min.

In this sort of situation would there be any practical need/advantage for a coilgun to have multiple barrels?

Many thanks.

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    $\begingroup$ Crowd control with missiles? That sounds humane. It would be better if each barrel pointed a different direction, I don't see much benefit to having multiple barrels pointed in the same direction unless there is an overheating problem to avert. $\endgroup$ – ohwilleke Nov 16 '16 at 22:05
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    $\begingroup$ @ohwilleke Gatling gun barrels are actually super aligned with each-other, and the weapon systems themselves are extremely accurate, especially under continuous fire. Often time gunners will "walk" their fire to their target once they see where the bullets are actually going; it functions a lot like a hose or how one would imagine a continuous laser weapon would. Its easier to get on target if you have the constant feed back of hits rather then "-pop did I hit? pop did I hit? pop-". Its more "BRRRRRRRR-ah, to the left a bit-RRRRRRTTTTT". But I agree on the "humane" part $\endgroup$ – Marky Nov 16 '16 at 22:36
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    $\begingroup$ Sorry when I said crowd control I didn't mean civilian, and "control" should be taken with a mountain of salt. I guess more funnelling, through death. $\endgroup$ – mattolf Nov 16 '16 at 23:04
  • $\begingroup$ I think the term you were looking for is "anti-infantry" $\endgroup$ – Shadur Nov 17 '16 at 14:30
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    $\begingroup$ I'll invent the phrase anti-small-medium-mutant-bestie $\endgroup$ – mattolf Nov 17 '16 at 17:30
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A couple reasons that can work together or separately for this to happen:

  1. Heat management
  2. A normal "machine gun" style gas operated piston is unsafe for this weapon for some reason

Heat

One of the reasons modern vehicle mounted weapons like the GAU-8 have multiple barrels is not for any sort of aesthetic reason but for heat management. Even then, the GAU-8 cannot be fired for long periods because it will still overheat. It has a firing rate fixed at 3900 Rounds per minute.

Weapons with high firing rates tend to overheat rather quickly using chemical propulsion.

Modern Infantry machine gun teams must limit their firing rate and often, a spare barrel will be carried for squad automatic weapons. (One example is the m249, a squad support weapon used by the united states military, read about it here if you want )

Chemically propelled firearms will overheat eventually and if they are fired while overheated it can ruin or deform the barrel, rendering the weapon inoperable or unsafe.

If your weapon is a coilgun which uses chemical propulsion to provide an initial burst of speed to projectiles, this might be good enough for you.

Any weapon with such an extreme firing rate will be multi barreled in current technological application partly because of heat, but also because of the second reason.

Firing Mechanism

If your weapon is not using chemical propellants, you cannot use the same standard mechanism for automatically loading the next bullet as a modern machine gun or automatic weapon uses.

Modern automatic weapons use a portion of the gas produced by the burning of chemical propellant to drive a mechanism that automatically ejects an expended cartridge and allows a new one to be fed into the mechanism. This is a really high level overview as there are multiple types of automatic reloading mechanisms (Read here for more on different types of automatic loading in modern firearms)

Automatic weapons which do not use this mechanism to load another round would require another method to load the barrel safely and reliably. One of these could be using multiple barrels. In this way, because the barrel being loaded is not actively expected to fire the next round, there is more time for a mechanism to properly load the projectile. The Gatling gun is one example of this type of weapon and was used in the United states civil war and other conflicts.

Therefore it is safe to say, firing mechanisms may be the reason your weapon has multiple barrels.

For example, if you had a railgun and not a coilgun, you could say you needed multiple barrels to ram the projectile into place adequately.

The key here is the safety and reliability. Machines in real life cannot operate instantly and as a result there is a sort of upper limit on how fast you can safely reload a single barreled weapon. Perhaps the reason your weapon has multiple barrels is because it is the only sensible way to load rounds in a reliable way.

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    $\begingroup$ There are some non-gas-operated guns in real life that have a single barrel, the Bushmaster is one that comes to mind; but they do generally have a lower rate of fire. $\endgroup$ – 2012rcampion Nov 17 '16 at 3:51
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Historically, there were two reasons to have multiple barrels.

  1. Rate of fire was higher than rate of loading. Extreme example - systems that can only be loaded in base. Without extremes - gatling gun was a solution to loading problems. Now this is long gone for regular guns, don't know about rail or coilguns.

  2. Temperature. If you have 6 barrels, each one receives only 1/6 of the heat. This greatly increases reliability and allows simpler construction, without radiators and other kinds of cooling. Example: M61 Vulcan

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    $\begingroup$ The temperature thing is super important for rapid rate of fire. $\endgroup$ – jorfus Nov 16 '16 at 22:26
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    $\begingroup$ This. with 6 barrels you have five times the time it takes you to actually fire one barrel to get the next reloaded. Variants exist with different numbers of barrels; 3 is depicted in movies semi-often, and the gau-8 has 7 (the a-10s gun). The reason we dont get super fast fireing tank cannons is partly because they get to maximum heat limit just on muscle power already. A Gatling configuration would solve this problem (but who needs a 120mm Gatling gun anyways.) $\endgroup$ – Marky Nov 16 '16 at 22:29
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    $\begingroup$ @Marky "but who needs a 120mm Gatling gun anyways" Someone who can never have enough dakka. $\endgroup$ – JAB Nov 16 '16 at 23:41
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    $\begingroup$ BTW -- part of the reason the M61 and its progeny replaced revolver cannons such as the M39 and conventional linear actions such as the Colt Mk 12 for US aerial gunnery duties is because of loading issues -- a Gatling gun will just dump a dud round back in (or out) with the spent casings, while revolver and linear-action cannons can experience a feed stoppage or jam if a round misfires as there won't be any energy to eject the dud and replace it with a functioning round. (And that's on top of the gun being potentially unsafe to fire!) $\endgroup$ – Shalvenay Nov 17 '16 at 3:05
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  • The process to transfer energy from the electrical power plant to the projectile won't be 100% efficient. The waste energy will become waste heat. Multiple barrels help to cope with this excess heat.
  • It may be feasible to share components between coilguns of different caliber. The same power supply could be used to fire a relatively large but slow mortar round, a smaller but faster autocannon round, or plenty of MG rounds. If the power supply is the heavy or expensive part of the weapon and the barrel(s) and ammo feed are cheap, it may make sense to share the power supply.
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one problem with current railguns is wear and tear on the barrel, being able to swap out barrels might be a solution. if you would only get 8 shots per barrel, having six barrels gives you 46 shots with the same machine.

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Besides the heat- and loading time problems already mentioned by others multiple barrels with separate loaders make it easier to fire different kinds of ammo from each barrel.

Your walker might have one barrel loaded with the coil gun equivalent of shot (a steel casing full of steel balls with the casing falling off after firing). The second barrel with ke-penetrators and the third with whatever you might come up with or the same as one of the other barrels.

Mixing ammo might cause problems with just one barrel even if the barrels have an identical setup because loading times or acceleration by the coil might vary.

When going on a anti infantry run all the barrels except for one might be loaded with ammo meant for soft targets but you'd still be prepared if some hard targets appear.

As a side-note: if this monster is meant as an anti infantry vehicle I'd build some flamers into the feet in case someone gets close.

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It's as simple as more barrels = higher rate of fire. You can fire them all at once (fragmenting shells would make a wall of death, or AP shells for armoured targets), or sequentially to take out individual targets. You'd either have a rotating set of barrels loading one at a time (like a minigun), or have fixed barrels with their own loaders mounted on a turret (Like a double barrel shotgun). maybe one on each side for versatility?

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  • $\begingroup$ not to mention you have reduced wear on an individual barrel, meaning 3 times as many rounds on target before it has to be repaired or replaced, each individual barrel won't heat up as much as one barrel firing the same number of rounds (because they are firing 1/3 of the rounds each), and you have more options for amount of firepower to put down. You could always have it running on "single rate" mode and crank up the juice when the going gets hot $\endgroup$ – Marshall Tigerus Nov 16 '16 at 21:42
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When "combating large hoard like enemies" don't flock shoot.
You can track targets based on pattern recognition (face detection), movement, and/or heat signatures. Give each barrel a 10 degree field of fire and let them help aim for you.

I'm thinking of the last Matrix movie.

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    $\begingroup$ While this provides some helpful information, it may not be substance enough for a complete answer. Consider reposting this as a comment to suggest this idea. $\endgroup$ – Zxyrra Nov 17 '16 at 3:36

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