Another possible material would be metastable excited nuclear isomers. You’d need to make them go off somehow. I don’t know much about this but I did a very rough back of the envelope for one example and got 50 GJ/kg, which needs to be multiplied by whatever fraction of the projectile is actually made of it. There would be some radiation. A while back there was a physics research group that claimed to be able to make a graser out of this stuff but I don’t think anyone confirmed their work.
It was definitely not confirmed, although the Pentagon spent a lot of time fussing over it.
A general thought: the power of a modern assault rifle depends at least as much on the materials science that went into making the barrel as it does on the energy density per unit mass of the propellant.
If someone made a gun propellant twice as powerful as what we use now or would be adopted, the amount of propellant would be halved, that would save weight, and as a result we would be able to give our infantry slightly more powerful weapons. But the effect wouldn’t be dramatic, since most of the mass of a rifle plus ammunition isn’t propellant. OK, that deserves qualification if you’re an infantryman who’s yomped for days with it on your back. But for most PCs.
If, in the other hand, someone made a rifle barrel with twice the strength per unit weight, that would be awesome. Because a lot of the weight of the weapon and ammunition is proportional to that weight. And by instead increasing the power and keeping the weight constant, something considerably more effective could be made.
An important measure of rifle efficiency is muzzle energy divided by rated pressure of the barrel divided by barrel internal volume. Designers manage to get this number surprisingly high. Because it matters more than explosive yields.
If you keep the weight constant but boost the muzzle energy, aren’t you going to have boosted recoil too? How much momentum you have to cope with seems to be a limit on your ability to use a firearm.
I don’t think so. Unless maybe if you’re firing long manual bursts on full auto, and modern soldiers rarely do that unless the weapon is stabilised against recoil. The Brown Bess musket, for example, had maybe ten times the recoil energy of a modern assault rifle, and people coped. Also you can make the projectile faster and lighter, which saves you some recoil, if that’s important to you. I think the mass is a bigger limit: how much can you carry around, and point accurately.
There are various tricks (the videos on forgottenweapons sometimes mention them) - if you’re actually designing for sustained automatic fire, you can tweak the firing cycle and countermass so that you get a steady push against the shoulder rather than thump-thump-thump. But nobody bothers to do this at the moment because, as David says, that’s not a common mode of use for small arms in current tactics - from that perspective, rifles are mostly for making the enemy keep their heads down and stay in one place while you get the machine guns or mortars or other frightfulness onto them. You may kill or wound a few of them, but you don’t expect to have a battle that ends with one side running away or being wiped out if all you have is rifles.
The cynic in me says that if you make the issue rifle lighter for the same capability, the soldier will be loaded down with more other stuff until they’re carrying just as much as before. (This has been a major concern and is one reason why there was a big parachute procurement exercise a few years ago - the dropped mass of a combat soldier has gone up by quite a lot since WWII. The other reason was that there’s a lot more variation in that mass now, and if you have one-size-fits-all your lighter soldiers get literally blown away.)