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After getting very good answer concerning which kind of weapon would be reasonable on a planet with air density of 3 atmospheres

(What would be a reasonable caliber for an assault rifle and sniper rifle for a planet with pressure of 3 atm?)

and doing some googling, I started to analyse this concept a bit deeper.

So effectively I end with people armed with flechette sabot stabilized guns, which ammo has comparable size (but some specially designed ammo seemed a bit longer than one designed for conventional bullets), is smootborne, has extraordinary aerodynamics, extraordinary armor piercing quantities and... awful terminal ballistics (which means that pierces enemy but tend not to do much actual damage).

As such it would be an acceptable trade off. But in close combat situation long range properties are not an issue while stopping power matters. And over penetration is not always advantage... How should it be solved? Especially that not rifled weapons tend to have low accuracy.

(Normal bullets for short range and accept this low accuracy? Should there be some fattened sabot stabilized projectiles? Shotgun like slug bullets with their own "rifling"? Or maybe such different kind of ammo would be too problematic, and just using a short burst would solve any problem of inferior terminal ballistics? Or just submachine guns if someone really plans and needs some house to house combat or hostages saving and otherwise just normal guns?)

Should there be proper short distance ammo? If yes, of which type?

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  • $\begingroup$ These design considerations are the reason why the US military experimented with flechette ammunition and ultimately rejected the idea. $\endgroup$ – sphennings Mar 28 '17 at 14:38
  • $\begingroup$ It's worth noting that you can use a sabot with a rifled barrel, but it would probably be bad if you are packing more than one flechette in each sabot as they wouldn't be spinning on their center axis. $\endgroup$ – AndyD273 Mar 28 '17 at 19:26
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You check YouTube you'll find a large number of videos of people trying out different custom slugs.

Shotguns are smooth bore, though you can use a riffled slug. But having a big, heavy slug will do more for accuracy on the short term.

Also for close range a normal shotgun load of small heavy pellets is pretty good, since the spread makes aiming less important.

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For short range, why not use shot instead of single rounds? A shot round spreads the kinetic energy over multiple projectiles, transferring more of it to the target. Smaller shot means less energy per shot, so the individual BBs don't necessarily have enough energy to punch through the target.

Soft shot (copper, lead, aluminum, etc.) would flatten on impact without sacrificing your aerodynamic cross-section. The flattening would transfer more of the kinetic energy into the target, improving stopping power (as Paul said above).

Lightweight, strong shot would ricochet inside the target, causing additional internal damage.

Brittle shot (or even a brittle slug) would fragment when it strikes something hard (like bone). This would be less effective against solid armor, though, as the round would have it's own ablative effect, dispersing its energy into its fragments.

As a bonus to all of this, many modern assault rifles can have a compact shotgun mounted to them, so you needn't necessarily carry two complete weapons into a mission.

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You could look into some of the solid copper ammo out there. There are rounds that are solid copper with a hollow point capped with a small amount of plastic to fill the hollow. this means that the round leaves the barrel maintains it's aerodynamic shape but then deforms on impact.

With normal bullets on this planet, hollow points are the ones that have the best stopping power (generally speaking). The effect of the expansion of the bullet is cavitation, shock wave propagation in the body, and ending with an exit wound the size of a fist.

So, slightly larger slug, soft metal, and hollow with a capped point that is designed to expand on impact. There is your stopping power.

You can look around and find some modern day handgun ammunition and find some terrifying variations on this theme.

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The trouble with flechettes is that their light weight would cause them to lose speed very rapidly due to air resistance. This will be an even bigger problem at 3 atmospheres. What you need are the heaviest slugs with the smallest cross section you can get. Depleted uranium would be best, if you can get it.

If memory serves, they tried flechette ammunition for shotguns during the Vietnam conflict and determined that they were very good for removing leaves from trees and that's about it.

There might be a more effective modern equivalent. Small, sharp, teflon coated sub munitions might be good for getting through kevlar if there are no inserts but hard armor would just bounce those off.

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