"Infantry" main weapons for the setting described below? (NOT a duplicate for reasons explained in the post)

Question

(before anybody decide to flag this post as a duplicate, there's an explanation of why it looks identical at the bottom of this post)

what would be a suitable choice for "infantries"(the quote unquote is because it's not necessarily a real person, possibly just a rover with a gun doing the job of a foot soldier) for this following setting? the general background, especially the technology, is a bit messy with a mix of near, mid and far-future tech that I'll attempt to outline below (for example a capital ship in the setting belonging might have a antimatter beam-core engine while it's missiles are methalox/Metallic Hydrogen rockets launched from a coilgun and communicating with other ships with lasers then getting a reply with neutrinos)

assumptions:

1: The function of the civilization being discussed (hereby referred to as the state)'s infantry is much like today's, namely fighting in buildings, tunnels, forests etc where armor and artillery is either unavailable or need their support, alongside with holding these regions and general low-intensity engagements

2: The opponent is also human, however as they're interstellar colonists who had no contact with the state for roughly three hundred years, the state is as familiar with them as the Aztecs are familiar with the conquistadors. They relies on infantry far more than it does on armored vehicle or artillery (if this sounds awfully world war 1 to you, you're absolutely correct, because the vast majority of their wars are either purely space combat or really didn't call for more than infantry). They have little automation so robotic enemies are out of the question. Their infantry is protected either very poorly with little to no protection at all, with heavy armor that would be considered NIJ standard type IV armor by modern standards, or force fields that could be depleted with 20 hit from 12.7s while being armed with star wars Blaster-like weapons. Their force fields could stop a single EMP or electrocution attack by taking it for whatever it is protecting and getting disabled as a result.

3: The infantry (alongside with anything that is intended to be used on the surface) is deployed using Project Orion style dropships capable of landing with large payloads around 200 tons. However due to the fallout of detonating nuclear devices as thrust, the infantry may only leave while inside their IFV or APCs (which also means that unless otherwise specified, the infantry is always expected to have their vehicle somewhere nearby to support them)

4: The state as a whole utilizes only technology that would be considered unobtanium at most (for example a Kerr-Newman black hole on a vessel is fair play but a lightsaber is not), handwavium is avoided with the possible exception of negative mass

5: while most of the fighting is on shirt-sleeve habitable planets, it is also required to be able to function in non-habitable environments such as a methane atmosphere (though it does not have to function in vacuum)

6: The state considers it's army (space force not included) to be largely expendable in the sense that they would try to force the enemy into attrition warfare if it believe it could win one, does not care much about public relationships (this is after all a totalitarian regime), and draws from a larger overall manpower/resource pool than it's opponents possibly could. However when on the offensive they tend to have less units immediately available due to having rather expensive transport costs, therefore superior individual performance is desirable

7: the state has very capable and portable power sources that could be scaled down to infantry use in the range of 10-30 megajoules per kilogram

8: Powered armor(or exoskeleton, depending on how yo classify them) exists, they could provide a large amount of power to the soldier's equipment if it requires any, and the state could afford to equip a large amount of it's soldiers with them. However they are much less capable than Battle Brothers or anything like that. They are capable of boosting mobility and firepower but anything that will cause a individual soldier to be heavier than 235KG will not be accepted, in order to not collapse the floor/stair they might be standing on (if they can't enter a building safely than there's no reason for them, why not bring in a tank?). They only provide armor in critical areas such as the chest, and no radiological protection beyond what is expected from a HAZMAT suit (although this does make them impervious to chemical weapons). On the other hand they are notably un-bulky compared to the traditional image of powered armor, appearing simply to be a slightly higher soldier (the closest example I can think of is Iron Man)

In response to the situation being described above, what could the state's infantry be primarily armed with? (just the "assault rifle", more specialized weapons such as anti-armor is a totally different issue, which I also happen to be more confident in)

(Disclaimer: This post looks near-identical with a previous post that I made, because the previous one started off as a ridiculously unfocused question(scroll down to see how horrible the original one was). It was closed for understandable reasons after commentors explained to me why it is a terrible post, so I edited it into the current shape that you see above, however there is not enough voters to reopen the original post (at that time, at GMT june 19th the original post was reopened). I still need a answer, since the previous post was closed before any agreeable answer came by although the suggestions on how to make a good question did get incorporated, but I can't just delete a post with upvoted answers either. So after flagging the original post and getting advised by either a moderator or a senior member to repost it with clear explanations, here we are)

Answer 1

Aside from the obvious questions around "why are you using infantry" and "why aren't you using appropriate weapons for the environment; surely an interstellar multi-planetary industrial base for a civilization apparently happy to wage war on a similar scale can manage a reasonable logistics chain", it seems to me that the most general-purpose weapon format is probably small self-propelled projectiles.

It is hard to tell exactly what sort of tech-level you're after here because your setting is so inconsistent... beam-core rockets side by side with methalox rockets? Do your ground forces use stone-throwing artillery, still?

Anyway, self-propelled projectiles get a bit of a bad rap in the real world because of the failure of gyrojets, but everyone seems to forget that they were invented and trialled in the sixties and technology has marched on a bit since then even in the real world, let alone in a warlike interstellar civilization.

Lets imagine these small rockets are under 20mm in diameter. You probably want more like 10mm for strictly antipersonnel use (less overkill, easier to carry more rounds), but you might have multi-purpose double-barrelled weaponry, or squad-level heavier weapons, maybe with larger-bore projectiles.

Micro-missiles offer a number of useful features, such as:

• can work in a wide range of atmospheres, including no atmosphere at all.
• reduces complexity of gun, making it more robust especially in the presence of hostile atmospheres.
• works even in opaque atmospheres (eg. dust clouds, fog, smoke) which seriously hinder beam weapons.
• no dependency on high-capacity, high-discharge-rate power packs.
• minimal cooling issues.
• minimal recoil, making it convenient for use in microgravity environments.
• potential to mount a variety of different warheads, or use kinetic penetrators.
• potential to do mid-course guidance.

The latter two are the most interesting ones, and where this diverges from the Space Marine style. Being able to do stuff like beam-riding, or autonomous target tracking, or simply replay a course programmed at firing time gives you a very versatile weapon that can do a bunch of novel things that regular projectiles and beams weapons cannot:

• limited ability to fire "around corners"... useful in some kinds of rotating-wheel style space station, but also useful for trajectory correction in any kind of environment where artificial gravity is provided by rotation, and coriolis effects make regular projectiles much harder to use (I've written an answer on this subject in the past).
• ability to trade off range to maintain a flat trajectory to make aiming easier, even in bad weather, very thick atmospheres or under elevated gravitational fields (or on spacecraft undergoing high accelerations).
• ability to trade off projectile velocity for stealth, keeping rounds subsonic over long ranges, hindering detection of the shooter.
• potential for beam-riding guidance for very high accuracy, even against (probably slow) moving targets.
• separate spotters or targetting drones or devices let users shoot targets they can't even see.
• active warheads of various kinds mean lethality is the same at any range, regardless of projectile velocity.
• ability to air-burst a fragmentation warhead at a specific range to attack targets in cover.
• armor-piercing warheads for tough targets like light vehicles, or armored exoskeletons
• non armor-piercing warheads for use in fragile, hazardous environments like some kinds of space station or spacecraft.
• specialist warheads for specific circumstances, like incendiary, chemical weapons, EMP, miniature antipersonnel mines, flares, smoke, harsh language, etc.

The approach scales well with future technologies, taking advantages of improvements in miniaturized guidance systems or novel warhead types (fullerene-encapsulated antimatter, anyone? maybe an antimatter-driven single-pulse gamma ray laser?).

Loadouts can be tailored to the mission and the role of the person using the gun, perhaps with some emergency use special rounds to help cover a retreat from unexpected resistance, and so on.

I still hold that thinking about infantry at all is perhaps the ultimate "fighting the last war" in this situation, and your starship troopers would probably all be annihilated by autonomous antipersonnel drone swarms that will probably be coming to a real-world battlefield in the depressingly near future, but that aside: micromissile launchers seem to be sufficiently versatile that they can slot into a one-size-fits-all weapon category for your setting.

Answer 2

Fighting in atmosphere you need weapons the don't bloom that rules out most lasers, plasma, and most or all particle weapons, even the ultra relativistic electron beam that is famously low bloom, and low divergence, in a vacuum doesn't work in atmosphere. That leaves you with projectile weapons and lasers at frequencies that don't interact much with the air. That's microwave and gamma ray lasers, coil and rail guns, gyrojets and modern fire arms. All of those weapons are going to have issues in atmospheres with notable amounts of free Methane or Ethyne so I would think that any infantry weapons are going to have light weight blast shields, probably polycarbonate or similar, that can be fitted to them for operations in "reactive" atmospheres. The extremely high frequencies of the laser weapons is going to be a terribly inefficient use of energy. Coilguns have issues with complex timing, current transmission density, and thermal loading problems. Railguns have those same issues plus rail erosion (I have seen a cool schematic for clip feeding rounds complete with single use rails). Gyrojets have major ammunition density issues but could potentially make good failsafe weapons for use inside pressure hulls.

My first instinct is to go with modern firearms using caseless ammunition, it's a reliable and mature technology that works acceptably in almost any conditions, there are already types of ammunition and calibres that won't over penetrate, and that will penetrate human (or even tank) portable armour etc...

Answer 3

Firstly - Love the new question - I can see you took my feedback on board and the new question is much better.

The key things that jump out at me are:

1: Not expected to need heavy fire support (e.g. engaging enemy vehicles, Troops are not heavily armored, not a lot of bunkers etc.)
2: They are expected to be delivered in drop pods and then in an APC or other armored type vehicle.
3: Their enemy is expected to be numerous, but poorly armed and armored.
4: Power Armor is more of a specialist tool than standard issue.

So firstly - let us talk range - your combat environment is expected to be Buildings, Forrests and Tunnels - so for starters, we are going to be sacrificing Range - what is the point of having a weapon system that is lethal out to 10 kilometres when your sight-lines are at most 100 metres (adjust your actual sight-lines as necessary)

Next up we have our Enemy - Since we are fighting a lot of infantry who are lightly armoured - having smart weapons is less of a concern. Smart weapons have a cost to them (in terms of manufacturing and other) - you typically see that as the target value goes up, so does the smartness of the weapon system.

It makes little sense to use something like a smart homing rocket on some poor sap who has no body-armor, when a simple projectile or beam weapon would suffice.

What we are going to be considered about is the ability to make rapid follow-up shots, large ammunition capacity - and since you mentioned that the state is totalitarian - I am going to make the presumption that they take the Canadian View: Geneva Suggestions or the Geneva Check-list... So things like indiscriminate fire or collateral casualties aren't a concern.

If we are using projectile based weaponary, then we have to contend with recoil (which we don't want).

Next up is the weapon itself - this is an interesting one - as a primarily mechanized force, we have the possibility that we could have a weapon that is heavier than a typical rifle (This could mean more energy cells, more ammunition, more do-dads, or if we go the projectile route - heavier weapon = less recoil).

However, again, if we look at our expected areas of operation (Tunnels, buildings Forests) we want something small and nimble so that we quickly clear corners and rooms etc.

In terms of Power - your power cells - 10-30 MJ of energy per Kilo, an M1 Abrams tank round has a Muzzle energy 12 MJ, and the lowly .223 cartridge has a Muzzle energy of around 1.7 KJ - 1,000 rounds of .223 ammo is around 14 Kilos (depending on projectile weight) - so call 100 rounds 1.4 Kilos - doing some quick and dirty maths - a 1 Kg Power cell in your universe is a lot of .223 equivalent ammo.

Finally, we want a system that can be integrated with their APC to operate in hostile environments.

Mixing that all together:

We want a Weapon where the functional bit that goes in the hand is short and easy to point. We want minimal recoil and we want the ability to fire a lot. We are not super concerned with the over-all weight of the weapons system as we our expecting our troops not to be doing too much walking. We are probably looking at an Energy based system as we can get plenty of bang for a given weight. We aren't looking for Smart weaponary per se as that is overkill, we aren't worried about range either because our expected engagement distances are very short.

I think we would see some form of Back-pack mounted, flame-thrower esque (Plasma thrower?) system.

Being an all-electric system means that we aren't worried about things like explosives transport or ammunition cook-offs.

We could also have it integrate with the APCs like the old AK rifles did on the BMPs where the backpack component could be stored in the APC, and the nozzle part would be put through a sealed port.

The State isn't worried about things like War Crimes or indiscriminate damage - in fact, if it is totalitarian in nature - it might even favor the terror aspect of a high-tech stream of plasma vaporizing parts of an enemy (Psychological warfare is still a thing - Even on the modern Battlefield, if you hear the Brits call to Fix Bayonets, shit is about to get stabby).

Using a standard power cell would be great for logistics (Just like builders and their tools use a standard power cell) allowing 'ammo' to be shared between squadmates or even other areas like maintenance or engineering.

Finally - given your World War 1 inspiration having a modern Flammenwerfer suits the aesthetics (especially if you give your infantry gasmasks for Rule-of-cool)