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I am working on a series set a few hundred years in the future where fusion power is common and starships roam between planets. I am trying to think of a way to power small Arms weapons and keep them feasible, so no laser rifles or coil guns due to a lack of power storage and heat issues. (unless there is a plausible battery that could be developed to allow for a few hundred rounds (1000kJ ) and have be less than a kg).

Back to the point, my main idea is to stick with chemical propelled projectiles such as bullets (caselsss ammunition).

The question: Is there a potential chemical/energy source that could be attached to the bullet that would be more compact and lightweight (and have the same output) as a traditional modern charge?

The ability to fire in space is also a prerequisite.

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    $\begingroup$ An idea that comes up repeatedly is caseless ammunition since it makes the ammunition drastically lighter *thus allowing soldiers to carry much more) and makes the gun simpler, harder to jam, and faster to cycle. No one has managed to design one that did not have major flaws, but it is not inherently impossible. the G11 came close but the uncased rounds were to fragile and prone to overheating. $\endgroup$ – John Jun 3 at 4:13
  • $\begingroup$ Small point of clarification: gunpowder will work in space without any modification, because it contains both fuel and oxidizer. (Which is not to say that the guns that fire it will necessarily work.) $\endgroup$ – Cadence Jun 3 at 4:17
  • $\begingroup$ I'm not really sure why you're ruling out coilguns and directed energy weapons as implausible in a setting hundreds of years in the future where fusion power is a solved problem? On that timescale they seem more likely to still exist than traditional projectile weapons with chemical propellants. $\endgroup$ – Nathan Griffiths Jun 3 at 7:36
  • $\begingroup$ @NathanGriffiths Fusion power is a lot about scaling, just because you can build a fusion reactor doesn't mean you can build one that fits into a hand held weapon. $\endgroup$ – Whitecold Jun 3 at 14:58
  • $\begingroup$ Much as I love the topic, it has been done here a number of times. worldbuilding.stackexchange.com/questions/136965/… worldbuilding.stackexchange.com/questions/54859/… worldbuilding.stackexchange.com/questions/126898/… $\endgroup$ – Willk Jun 3 at 15:40
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Rocket fuel

If you want something more energy dense and something that can fire in the vacuum of space, just use rocket fuel, based on something like hydrazine. Rocket fuel mixtures, generally speaking, are self-oxidizing and rapidly and exothermically expand into gas, similar to gunpowder. Except it's more energy dense. It's also a lot more dangerous, which is why we don't actually use rocket fuel in modern guns.

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  • $\begingroup$ Good idea. Although dangerous it is possible to mix liquid oxygen and liquid methane, although such mixtures are shock sensitive $\endgroup$ – Slarty Jun 3 at 8:13
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    $\begingroup$ The other reason not to use liquid rocket fuels is that they are usually toxic, carcinogenic, corrosive and volatile. They can be handled, but there is a reason why armies prefer solid rocket motors whenever possible nowadays. $\endgroup$ – Whitecold Jun 3 at 14:48
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Air

"You can have a fully automatic gun that shoots lead but without gun powder."

Below is a photograph of the Seneca Dragon Claw, a .50 caliber rifle powered by air. And the quote above is from a review of the Dragon Claw. This particular rifle is a single-round breechloader, but you could have an air-powered combat rifle and it would definitely work in space.

Photograph of air-powered rifle

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    $\begingroup$ If you use this in space, remember to bring some air with you. Actually if you do anything in space, remember to bring some air with you. $\endgroup$ – Willk Jun 3 at 13:49

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