Ok a gun for hard vacuum use....

Frist off, standard rifle ammo, or any kind of power ammo will work perfectly fine in space. The chemicals of smokeless powder contains its own oxygen.

It needs brass or another metal from the main gun's receivers as at least a liner on everything that slides against itself because cold-welding is HUGE problem in space. Because of it you always want dissimilar materials touching each other. Likely the rounds will be coated in a light grease or something as well for the same reason.

Your barrel is going to be really chunky, and likely covered with a large aluminum heat sink as well. Remember in space there is no air to take the heat away from the gun, so the GUN has to soak up the heat and just kinda hold it. Meaning it will get really, really hot if it doesn't have a bunch of mass to soak that heat. It will radiate it slowly, but MUCH MUCH more slowly than a gun being cooled in air.

I believe a standard design gun like an AR would reach glowing-hot in 2-3 magazines, even at a modest pace of fire.

Stock is going to have heavy recoil dampeners to help off set the recoil from sending you spinning while space walking. Think about trying to shoot a gun while floating. Newton's 2nd law will see you spinning like a top after as little as 2 rounds with a normal stock. Though the military might opt for "gyojet" style ammo to move more in the direction of recoil-less rifles, were the bullet propels itself because of how big a problem recoil will be in space. Think about it, even if you had "mag boots" locking you down to the hull of a space craft, the impulse of firing a rifle would have enough energy to nudge and rotate even the whole spacecraft into a new heading.

Probably going to want a fore grip to keep the glove away from the barrel given hot+polymer space suit = holes in suit. Vertical grips are more accurate than side grips.

Also drum and C mags are just a place for things to go wrong. There is a reason why militarries all use straight magazines or belts. Drum mags jam.

Also really big trigger guard loop to fit space suit gloves, and actually a slim grip since the glove its gonna take up most of the palm of the space warrior's hand.

Ok a gun for hard vacuum use....

It needs brass or another metal from the main gun's receivers as at least a liner on everything that slides against itself because cold-welding is HUGE problem in space. Because of it you always want dissimilar materials touching each other. Likely the rounds will be coated in a light grease or something as well for the same reason.

Your barrel is going to be really chunky, and likely covered with a large aluminum heat sink as well. Remember in space there is no air to take the heat away from the gun, so the GUN has to soak up the heat and just kinda hold it. Meaning it will get really, really hot if it doesn't have a bunch of mass to soak that heat. It will radiate it slowly, but MUCH MUCH more slowly than a gun being cooled in air.

I believe a standard design gun like an AR would reach glowing-hot in 2-3 magazines, even at a modest pace of fire.

Stock is going to have heavy recoil dampeners to help off set the recoil from sending you spinning while space walking. Think about trying to shoot a gun while floating. Newton's 2nd law will see you spinning like a top after as little as 2 rounds with a normal stock.

Probably going to want a fore grip to keep the glove away from the barrel given hot+ploymer space suit = holes in suit. Vertical grips are more accurate than side grips.

Also drum and C mags are just a place for things to go wrong. There is a reason why militataries all use straight magazines.

Also really big trigger guard loop to fit space suit gloves, and actually a slim grip since the glove its gonna take up most of the palm of the space warrior's hand.