What does the inverse square law have to do with lasers?
I think you are right about this, though- the power deliverable as momentum by a projectile will beat a laser of comparable size and mass for the foreseeable future.
I agree with all of the practical objections to lasers, particle beams, etc. brought up so far, and agree that firearms are the only really feasible tech for a hand weapon with real-world technology, but something you also have to keep in mind is that ammunition weighs a lot in any kind of quantity.
An individual round of current rifle ammo is about 12–13 g for 5.56, 18-20 g for 7.62. A single full magazine can range from about 450–800 g, depending on capacity and materials. A typical infantryman loadout is 20–30 30-round magazines. That does not include resupply, the firearm itself, other equipment, etc. You can see that you’re looking at significant mass for conventional firearms, and unless you’re postulating inertialess drives or some other magical technology, mass is extremely expensive to move around.
If firearms were used on spaceships or habitats, I would expect them to be a different design. For instance, the advantages of brass cartridges in ease of manufacturing would be greatly offset by the weight advantage of caseless ammunition, despite the other difficulties that hold back adoption of that technology on Earth now. Even more mass-frugal would be using compressed gas as a propellant, which has the advantage of making use of equipment that would already be necessary in one form or another on a space-going habitat. Spring or tension throwers (crossbows, spear guns) of some kind would make a lot of sense too, despite being less than practical for most purposes on the ground.
Besides the mass problem, conventional firearms have significant disadvantages that would not make them particularly suited to space. Most lubricants currently in use would sublimate in vacuum, and if they didn’t, would probably freeze, melt, or degrade under extreme cold or heat, depending on your orientation to and distance from a nearby star. You’d have to use dry lubricants like graphite or similar; and then you have to deal with particulates in your life-support system, or attempting to completely seal the weapon. (The presence of particulates, come to think of it, is another strike against chemical propellants.)
Those are problems I can come up with off the top of my head. I’m sure a picky engineer type could probably come up with at least a half-dozen more problems I haven’t thought of.
IMO there will never be a sufficient man-portable power system to make lethal death-rays possible, so I think chemical slug-throwers will be with us for the far-forseeable future. We may eventually warp across the galaxy with ease, but we won’t be killing strange new alien species with phasers. If nothing else, guns that sling slugs of metal propelled at several hundred feet per second will ALWAYS be able to inflict fatal damage, are cheap and easy to make, durable, and easy to learn to use - so why mess with success?
Now ship-based weapons may be another matter…
One of the Soviet Space Stations was armed with a 20 mm cannon. They actually fired it, I think. Although it’s purpose was to keep out Americans not Aliens.
Cosmonauts are armed with pistols and shotguns. For the same purposes I suppose.
The cosmonaut gun wasn’t for use in space.
Me too - don’t get me wrong.
I got that, however, it’s just unlikely that the exact same style of things would arise afresh out of necessity - even down to the use of Wild-West style fonts for signage.
Maybe a visiting starship left behind a Louis L’Amour novel. 
I’m pretty sure it’s called “Project Odin” in the new CoD: Ghosts, which, btw, has a thing or two to say on the subject of guns in space as well:
You know, I initially strongly doubted you since I was under the impression than space weapons were a huge agreed no-no in international law, but you’re correct (except for the caliber - it was apparently a 23mm autocannon). So, my apologies.
Sez here however that they resolved not to fire the cannon whenever there were people aboard, presumably out of fear the kickback and vibrations might shake some important Stuff loose.
[URL=“Salyut 3 - Wikipedia”]