Challenge: Handheld Space Weapons

I've been lurking around here for a while, but I decided to make an account for this thread. Here is my entry:
Defacto Industries presents... the 10mm Infantry rifle system - VaK2 Automatic Vacuum Carbine. Features:
- No modding (but black box power module... more on that further down, also contains diamond)
- 50 round magazine system mounted in the rear of the carbine
- Accurate fire against 1m^2 sized targets beyond 2km
- Capable of punching through most lighter armours
- Can empty magazine in 3 seconds if necessary, enabling use as LMG or SMG.
- Can fire many magazines in quick succession before barrel becomes dangerously hot
- Complete system weighs in at just below 6kg

There might be some optimization left to do. I think that the bore radius could be adjusted, for example, but I liked the nice even 10mm calibre. Also, if range is not considered too important, it's possible to shave a kilogram or so off the design and still stay above 1km range for 1m^2 targets.
Images:
steamcommunity.com/sharedfiles/filedetails/?id=1103431956
steamcommunity.com/sharedfiles/filedetails/?id=1103431477
steamcommunity.com/sharedfiles/filedetails/?id=1103430624
steamcommunity.com/sharedfiles/filedetails/?id=1103430279

The black box generator is a 10W hand-crank (a quick google made it seem like that was reasonable) with a weight of 0.9kg. I put the efficency at 33%. The system is water-cooled, temperature is 313K. This represents not just the hand crank but any electrical systems... one would imagine that it would be a bit uncomfortable to fire the gun while cranking, so that system can be thought to include a 0.5kJ energy storage, maybe in a flywheel or something. Or maybe it makes more sense to have the system operate non-electrically, skipping the crank altogether. Anyway, I think that with these limitations, it was reasonable to include the black box generator.

You can probably get well under .9kg with a tiny polonium or cobalt-60 RTG.  Seems like a pretty practical choice overall for this scenario though.

The Holmium mod is essential for the laser efficiency -- this laser runs a bit over 30% efficiency, rather than the usual 4-5% you get with default materials. You can do as well as this / better with efficiency using Ce:LLF and gold, but you will need more radiators because that melts at a lower temperature.

Limit mods are required to make it really low power and compact -- 100 g is more U-233 than I wanted for the reactor, 500k is more laser nodes than needed for a laser quality of 3.00, and I wanted to bring the laser power to less than 100kW. I am also tempted to make this into a drone, but I would then change from using one single laser to 7 (or 20), in order to make it more difficult to have the laser mirror sniped out.

I think that economies of scale mean you are better off with larger lasers than many small ones at the same cost, but that is not something I am sure about. Try it and see what happens?

Oh, nice!  I tried to build one of these a couple times but couldn't get within 2x the mass limit with anything reasonable.  What's the mass/velocity/range of the projectile?

Proposal: Coilgun Assault Rifle

It fires very small iron projectiles (10 mg), fired every 1 ms, at 1 km/s.  The coilgun itself is dirt cheap; most of the cost comes from the power source (same as in the laser cannon).
No material mods, just multiple limit mods.

Mass: 3.49 kg
Cost: 103 c
Length: 1.05 m
Radiation: 3.46 W
Peak heat: 69.1 kW
Projectile: 100,000x 1mm 10 mg Iron round at 1 km/s
Reload: 1.00 ms
Range estimation (1 m²): 1.07 km
Automatic targeting turn speed: 557 °/s

Just for the hell of it, here's what I suppose you could call a benchmark.  This is a rough approximation (basically, the best I can manage without being able to mess with the barrel profile, chamber geometry, etc.) of a 5.56x45 NATO machine gun, such as the FN Minimi or even a simple M16.  If you want an M4 just chop about 13cm off the barrel.