Caseless ammunition is maybe a closer thing, but also tricky (heat dissipation is harder, difficulties in making ammunition robust enough). H&K made a military variant once, but no military adopted it (Germany almost did, but with the end of the cold war it wasn't considered worth the cost)
Even adopting improved versions of existing systems is a big process for militaries, switching to something entirely new for something as fundamental as the standard rifles needs to have really big and obvious benefits to be considered.
Welcome to 1944.
This is exactly why full power rifle cartridges were replaced with intermediate cartridges like 7.62x39 and 5.56x45.
Kinetic energy is only loosely associated with effectiveness, terminal ballistics matters just as much if not more. In the real world intermediate calibers kill people dead enough reliably enough that it's a non-issue and marginal decreased effectiveness is outweighed by increased capacity. The overwhelming majority of shots fired will hit nothing useful anyway.
Case-less ammo was tried (read up on the H&K G11) for slightly different reasons (weight saving was a goal but not the primary goal) but the cold war ended before it could make it into service.
You are basically describing this[1][2][3] and it's not really that great at anything in particular (especially not military use) and are primarily used by people who want to shoot a trophy buck that's 400yd away. IMO they're kind of the hipster beer of the rifle world. They do little a boring old Bud won't do but they sure put a bigger hole in your wallet.
Read up on the history of intermediate cartridges[4] and assault rifles (not "assault weapons", the former is a technical term, the latter is anything but). Earlier intermediate cartridges (were a standard size .30-ish cal bullet on top of a 30-40mm case instead of a 50-70mm case of a traditional full power rifle round. Later intermediate cartridges such as 5.56x45 and 5.45x39 (the Russians stuck a smaller bullet on top of the 7.62x39 cartridge) stick an even smaller bullet on top of an intermediate size case because the smaller bullet going faster does the job just as well with less weight (I'm painting with a really broad brush here). These 2nd gen intermediate cartridges are what I think you're getting at but you're hung up on the rifle cartridge. For simply killing people 0-350yd away you don't need that much bang, it's actually detrimental because it's extra weight and makes the firearm harder to handle.
You also need thinking in terms of energy. All the energy in the world is useless if you can't transfer it to the target properly. It doesn't take much energy dissipated in a human torso to immediately incapacitate and kill. Better understanding of the needs of combat troops and nature of engagements and terminal ballistics is what led to the switch from intermediate cartridges starting in the 1940s.
Of course you have to remember that everything is still a trade-off in cartridge design and the nature of the engagements you expect still plays a large role in what the "best" tool for the job is and often the difference is only marginal at best.
[1]https://en.wikipedia.org/wiki/.270_Winchester_Short_Magnum
[2]https://en.wikipedia.org/wiki/.270_Weatherby_Magnum
[3]https://en.wikipedia.org/wiki/.223_Winchester_Super_Short_Ma...
There have been ongoing efforts for years, but there are significant technical obstacles to producing reliable, effective ammunition with reduced weight.
https://en.wikipedia.org/wiki/Lightweight_Small_Arms_Technol...
The whole concept of .223 is to put more propellent behind a smaller projectile. This gives the projectile more speed which makes it exceptionally lethal and accurate at very long distance. Unlike handguns that basically poke a hole, .223 generates a shockwave when it strikes at a multiple of the speed of sound.
Running just on the math, it seems the answer would be "of course":
E_k = 1/2 m \* v ^ 2
But we're not only concerned with kinetic energy. Momentum also matters. Or, in shooting parlance, "power factor", measured in grains (1/7000th of a pound) x velocity in feet per second. Momentum has a much closer correlation to recoil than kinetic energy. It also affects ballistics - a hypersonic lightweight round will have a different curve than a dense, merely supersonic traditional round.
Other practical concerns are also relevant: lead is an excellent lubricant/bearing material and aluminum oxide being an extremely abrasive cutting material...lead, bronze, cast iron, and other dense metals are more amenable to firing out of an interference fit with a barrel than light metals (of which aluminum is just one example, but a fairly representative one). Lead is highly ductile and mushrooms on impact, but aluminum is brittle and shatters on impact. A round fired at higher velocity spends less time in the barrel, so you need more powder, faster powder, and longer barrels to accelerate it more quickly. And the bullet is not the only thing ejected from the barrel: There's a supersonic jet of propellant gasses that contributes to both recoil and noise.
500 years of (un)natural selection in the field of ammunition materials science has left lead at the top of the charts for a good reason. Most lightweighting efforts have focused on reducing weight in the brass case, which is just packaging and becomes extra mass.
Finally, a combatant worrying about paperwork says something about the one-sided nature of recent wars: Million-dollar soliders applying overwhelming force and unimaginable funds against desperate guerillas. I wonder how quickly the slow-changing military will be able to respond if there's a war between superpowers.
Modern projectiles are jacketed so lead never touches anything.
If bullets are self-propelled, they are slower and less stable in the initial portion of their flight, which makes them much less accurate and gives longer time of flight assuming same top speed (which, for a similar quantity of propellant, isn't reasonable anyway; having a gun barrel is an advantage here), rocket and rocket-assisted projectiles work well enough when you can afford less inherent ballistic accuracy because of large targets or self-guidance, but neither of those is really practical for infantry smallarms.
If you look at the size of a common rifle projectile vs. a conventional cartridge, you’ll recognize that you’d need enormous energy density for any such booster system to do anything noticeable at all besides screwing with the reliably, safety, and stability of the round. A
Wouldn't it be possible to carry the same amount of kinetic energy with lighter ammunition? One inherent limitation of firearms is that all the energy is released inside the weapon, which has, then, to dissipate the heat and remaining gases and particles. If bullets are self-propelled, a larger part of that output can be released outside the weapon, allowing it to be faster and impart more energy on impact. Unfortunately, it may make following the source of the bullet easier.
> avoid the fallout associated with losing Army equipment.
That a combatant needs to worry about paperwork is the cruelest thing I heard in a long time.