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Here's the gist, my power armoured marines are going to be equipped with full-auto 8 gauge shotguns (in other words a quarter bigger then your average 12 guage shotgun). Now the marines are much more armored and heavily armed than their opponents but the marines are out numbered. Their enemies are colonists with basic knowledge of firearms and tactics. They have assault rifles and basic ceramic and Kevlar. The marines are very large as humans go and they are wearing power armor, so they will take up the entire corridor/lift. This makes it really hard to maneuver. The ship is pressurized but there is no gravity. With the these conditions and shotguns what is the best round:

  1. Buckshot. For all you laymen out there buckshot is basically a bunch of balls that are shot out of the gun. They disperse at range.
  2. Flechette rounds. A bunch of razor blades that are shot at the target. Kills you pretty nastily.
  3. Slugs. A chunk of metal that kills what it hits. Variants includes spreading brass and exploding varieties
  4. Incendiary. Burns things and sets you on fire.
  5. Other. Any kind of round that makes sense. Please say why you choose it and why other wouldn’t be as good. slugs and different types of buckshot

specialty rounds like sabot and dragons breath

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  • $\begingroup$ note unlike most bullets shotgun rounds are not airtight, and may not fire in a vacuum. I know you say it is pressurized but space marine may not be carrying them because of this. $\endgroup$ – John Jan 11 at 15:52
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    $\begingroup$ Why does being airtight matter at all? Gunpower has its own oxidizer. The shotgun would need vacuum-safe lubricants. $\endgroup$ – NomadMaker Jan 11 at 19:00
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Mixed munitions (including chemical weapons rounds):

I don't think you need just a single round, so this is a bit of a frame shift.

Okay, I don't fight fair. Your marines are in full-encompassing NRB suits, and your colonists are just, well, armored - and lightly at that. Now, you can't be sure the enemy will behave consistently, OR they don't have surprises. One of the great things about the future is you can play around with stuff. So here it is:

  • Guided gyrojet munitions/explosives: The lower velocities and very large bore of your shotguns makes them perfect to launch self-guided missiles, capable of using radar, heat, or even the emergency signals from a spacesuit to track opponents around corners, through doors, etc. Low-gravity environments are perfect for projectiles that have a guidance system and can self-accelerate when needed. They function equally well in gravity and in vacuum, so the marines don't have to use a completely different kind of weapon in different environments. Gyrojet projectiles can be fired down corridors and be programmed like tiny robotic ships to seek out critical infrastructure or specific targets (via facial recognitions, etc.) depending on how clever you want to make your weapons. Gyrojet projectiles can be all the types of weapons you are asking for here. You could even activate gyrojet munitions passively, toss them down a corridor silently by hand, and they would scan for opponents and fire their rockets when they detected someone. You could leave rounds behind you like an intelligent mine field waiting for enemies.

Let's consider projectiles like a toolbox rather than a choice. Your shotguns should have a dial allowing them to select ammo. Then a small "sawed off shotgun" version should be used for really specific projectiles (more of a launcher than a gun, really) like if you have an "assassin" bullet meant to seek out a specific target.

  • Shaped charges: I'd go with a miniature shaped-charge like a tiny RPG round for most situations. If I had to pick just one, this is where I'd go. This will blow apart doors, armor, etc. but once the explosion expands (killing whatever is on the other side) it hopefully won't completely destroy the station. Because it's an explosive, the projectile doesn't need to be going very fast to still be effectively armor-piercing. This also works well with the gyrojet idea of projectiles accelerating at opponents on their own (possibly at point-blank ranges).

  • Strong blistering/respiratory toxin rounds: Something like the high tech equivalent of phosgene oxime will be sort-of non-lethal, but really ghastly. It should cause rapid painful reaction on contact with skin and severe disabling effects when inhaled. So no mere respirator will block the crippling effects. Any enemy non-combatants become your instant hostages. Now all your opponents must don space suits (possibly shedding their armor in the process) to even stand up against you. Make the rounds spray the stuff in a cloud, or shatter on impact. They can be armor-piercing and inject chemical weapons through bulkheads like a giant syringe (to soften up enemies before you even enter a room).

  • Flechettes: For just shooting large numbers of weak opponents in a panic situation, I'd go with some sort of frangible armor-piercing multi-head sabot flechette. These work well also with the gyrojet concept (a single flechette can be a micro-missile, a cloud can be unguided). They can pierce armor while simultaneously minimizing damage to the station. The projectiles can be made less-lethal by making them not tumble but have a coating of paralytic agent (so after suffering a traumatic injury, the opponents swiftly become paralyzed). Having an unguided weapon as a backup means that if the colonists have somehow tricked your smart munitions, you can still pulverize them the old-fashioned way.

  • Capture rounds: Make a projectile that is a tiny claymore that maneuvers and faces their opponent. These can seek unarmored/lightly opponents and have tiny speakers on them, announcing their presence and demanding enemies surrender so you have instant hostages. The projectiles could even have superglue on them and land up against opponents (imagine a bomb landing against your mask and telling you to surrender or die), so they can't run away or swat the thing off. Detonates on command or at predetermined signals.

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Since you state that a marine will effectively take up an entire hallway during the boarding action, you're going to want to go with a round that can a) penetrate whatever armor (makeshift or otherwise) the colonists can muster and b) disperse over a short range without losing too much stopping power while c) not destroying the ship you're trying to take over.

Space ships tend to be cramped (maneuvering room is expensive!), so hallways will likely be short and curvy. You will want to pack ammo that can fill the space with deadly shrapnel, but not large enough chunks to compromise the integrity of the spacecraft you're boarding. Flechette rounds seem idea, given they utilize more than mass and power to inflict damage, while being small enough to not pose serious risk of damaging the structure. Incendiary rounds are about the worse form of ammo, followed by slug rounds. Those 2 types of munition will be as likely to kill everyone on board as to kill just who you want to kill.

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It depends on the goal you have.

Any buckshot, birdshot, flachettes etc won't be very effective against modern armor. However it's hard to completely cover people in armor, and in space you want to be as light as possible so not carrying full armor on the arms and legs for example is likely. This means that your full-auto shotguns could easily hurt the colonists quickly and potentially disable them.

On the other hand the space ship is likely some important real-estate, that's why you are boarding it rather than shooting it out of the sky which can potentially be done by dropping a damn tire iron in it's path at different orbital speeds. So spraying the inside with tons of tiny balls might not be useful as you'll destroy much of the space-ship you are in. Worse, tiny balls can easily cause electrical fires inside these ships so you are probably looking at alternatives.

Current shotguns don't seem to have a lot of oomph behind it besides their size. Yet there are options like making miniature discarding sabot rounds to pierce armor if necessary. Alternatively there already exist things like exploding shotgun rounds, basically miniature grenades, to either kill or disable the enemy with just a few hits. Since this is the future these rounds could be miniature shaped charge rounds, liquifying a bit of metal and launching it through the armor to pierce it and hit whatever or whoever is behind it.

If you are more humane, you could look into less-lethal rounds. From spraying the cabins and corridors with rubber bullets in the hopes of disabling the Colonists to more specialist rounds like small gas canisters with dibilitating gas concentrations or perhaps electrical rounds to taze the target (and again, full armor is likely not carried by colonists due to weight constraints).

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Small grenades.

Since the combat is taking place inside confined, pressurized metal cans, any explosion does not have an outside to dissipate to. As long as the explosion isn't powerful enough to rupture the hull, the shockwaves will bounce around multiple times, making mush of the colonists. Their ceramic and kevlar armor will be completely useless against blast shockwaves. Your marines in sealed hard suits will be well protected though.

As witnessed in the london underground bombing, explosions in confined places are very, very dangerous. The primary damage comes from the shockwave, rupturing eardrums, lungs and gut. Secondary damage comes from flying shrapnel. A third source of damage is from hot gasses which won't readily dissipate in the enclosed space.

Blast injury in enclosed spaces - NCBI - NIH: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC558684/

Air blast injuries killed the crew of the submarine H.L. Hunley: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5568114/

"People exposed to explosions in a confined space have increased mortality, higher Injury Severity Scores, more primary blast injuries, and more extensive burn trauma." - Conventional Explosions and Blast Injuries https://www.sccm.org/getattachment/cc197ca2-fe84-47c0-b3ef-7d00abd6271b/Conventional-Explosions-and-Blast-Injuries

Tear gas would also be very effective under these conditions.

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Shaped charge.

shaped charge

https://en.wikipedia.org/wiki/Shaped_charge

A typical device consists of a solid cylinder of explosive with a metal-lined conical hollow in one end and a central detonator, array of detonators, or detonation wave guide at the other end. Explosive energy is released directly away from (normal to) the surface of an explosive, so shaping the explosive will concentrate the explosive energy in the void. If the hollow is properly shaped (usually conically), the enormous pressure generated by the detonation of the explosive drives the liner in the hollow cavity inward to collapse upon its central axis. The resulting collision forms and projects a high-velocity jet of metal particles forward along the axis...

Your armored marines are basically tanks. In our world, shaped charges are used (among other things) to cut through the armor of tanks. The hole produced does not cripple the machinery of the tank, but produces a jet of superheated metal particles on the interior that kills the crew.

A tank round is only a little bit larger than your 8 gauge shell. Shaped charges like this would be for fighting other similarly armored marines.

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  • $\begingroup$ A miss with one of these will puncture the hull of the ship, and cause the colonists a lot more trouble. The suited marines won't be affected. $\endgroup$ – Innovine Jan 13 at 9:39
  • $\begingroup$ @Innovine, Innovine. Innovine! A spaceship is not a tank! Are you flying about in a Airstream motorhome, a thin shell of metal between you and the Empty, the walls of your ship frosted from your breath? Nay, man. Nay. You have insulated well the inner walls of your ship. You also used space for conduits and vents. Probably some posters of that artrock band that only you like, but you are the captain so ok. The shaped charge will spend its wrath on the poster, and the conduit, and possibly the insulation. It is not that strong, the shaped charge. It is focused. $\endgroup$ – Willk Jan 13 at 15:11
  • $\begingroup$ one would assume that armor is more durable than a standard hull. Hence the name. $\endgroup$ – Innovine Jan 13 at 17:49
  • $\begingroup$ @Innovine - with shaped charges armor becomes weapon. The small energetic jet carries particles of armor thru into whatever is underneath. Armor for a human cannot be that thick and humans hit by particles of armor will be injured. If charge hits wall, the small energetic jet carries particles of plastic, paper and drywall into the conduit space underneath, and some make it into the styrofoam insulation between that and the hull. If a particle makes it thru to the outside and leaves a mm sized hole it will whistle. The space marine will not be whistling. $\endgroup$ – Willk Jan 13 at 18:20
  • $\begingroup$ what is strongerr, spaceship hull (from the inside out, not counting whipple shields) or power marine armor? $\endgroup$ – Innovine Jan 13 at 19:28
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Tazer Shotgun.

It was deemed non viable because of costs. But if you are fielding Space Boarding operatives, you are well past making the costs trivial.

Tazer shotgun

You can render people safe, avoid over penetration, which is a huge deal on a ship. It also has 100ft of efective range, more than enough on tight ship rooms. Bear in mind that if your own troops carry power armour, this could potentially disable it, but won't kill it.

Nice for reducing casualties among your own due to Friendly Fire.

Now if your opposition grabs one...it could lead to an interesting plot hook. Disable the invader, take their tech armour for your own side...

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