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Revenge of the previous "gun guy" AMA

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ultra-modern tank cannons

This is in the context of small arms, though.

Personal body armor is now so incredibly good that the traditional final boss of anti-personnel armor piercing rounds, .30-06 M2AP, cannot penetrate it with a single hit. I'm relatively certain even .338LM's AP loadings can't get through either, and they can even stop non-AP .50 BMG rounds at sufficient distances (the actual AP stuff will still beat it). When the individual soldier's armor is shrugging off rounds designed to defeat light vehicles, and that armor is available to any industrialized nation at 200USD a plate I would very much agree that any military ignoring this is criminally negligent. Western militaries have all had the first-hand experience of their expensively-trained soldiers not dying is a big deal.

This armor is generally made of ceramic or plastic cells, where even if one cell is damaged it won't stop the other cells from stopping more incoming fire. So at that point, you either beef up your fighting rifle and cartridge to the point you can reliably get through that armor, or you fire more than one round in a burst that's fast enough to score a hit on the same cell of armor, defeating it.

The first approach is not a good one. More powerful rounds are heavier and larger, meaning the individual soldier can't carry as many- a problem once you've spent your entire load of 80 rounds and now you have no ammunition with which to close with and destroy the enemy- and they recoil a lot more so the weight of the gun has to be higher and affordances have to be made not to beat the soldier up too badly. Oh, and at that point your enemy then figures out that they might as well not wear armor if it's not going to protect them anyway (the norm for most conflict involving firearms) so you have these overpowered rifles that can barely keep a sustained rate of fire above that of a WW1-era bolt action rifle.

So we come to the second approach, which has comparatively few downsides. If the enemy decides not to issue armor after all (like Russian forces in Ukraine), you're not caught with some overpowered undersupplied monster rifle, it's still useful when it comes to suppressing fire (even if they know those rounds won't penetrate their armor they're still not sticking their heads into the incoming fire to find out!) because you didn't cut your individual soldier's ammunition supply by 3/4ths, and so on. It still depends on armor being defeatable in this way, and the individual soldier needs to be on target before they fire since it won't fix that, but we know that 6.8x51 at its public power level will still get through today's super-tough armor if you hit the same cell enough times- and there's only one known action that lets you feed/fire more than one round that physically big that fast.

Good points, in context of small arms, burst fire does make a good amount of sense.

Also, I'm suspecting there's a lot of things that haven't been tried. HEAT is probably not the best idea on account of the target being some sort of ceramic, but APDS would probably fit the bill nicely, as I imagine the plates themselves, when penetrated, do cause spalling, right ? APDS in tanks greatly increases penetration.

When the individual soldier's armor is shrugging off rounds designed to defeat light vehicles, and that armor is available to any industrialized nation at 200USD a plate

A plate. Which means, unless the guy is hit once or twice straight into a plate, he's just somewhat less likely to die. Which is very nice for the soldiers, but not really a gamechanger, as artillery is the bigger killer, and if they're shot at comprehensively, just as dead as an unarmored one.

Yet making a practical suit of armor that'd actually render the soldiers largely bulletproof to small arms, etc would still be prohibitively heavy, so you'd need either powered exoskeletons or bodybuilders and some cooling system.

APDS would probably fit the bill nicely

APDS in small arms has been tried before. The major problem with it is that a long, skinny dart doesn't have very good killing power- it isn't carrying very much kinetic energy and due to its nature of being a long, skinny dart has a very hard time actually dumping that energy into the target. Other problems include requiring much more precision in manufacture to fly straight and the sabots have a tendency upon separation to bounce off the ground at Mach 4 and injure squadmates (a problem noted in the SPIW trials).

If I recall correctly, exploding ammunition for small arms has some laws of war constraining its use. It's quite trivial to make an explosive 30-caliber projectile (InRange has a few videos about this, with both German and Russian examples used in WW2), but your accuracy suffers a bit unless you make the projectile correctly and the cost per round from both a pure BOM and manufacturing cost perspective increases significantly.

Which means, unless the guy is hit once or twice straight into a plate, he's just somewhat less likely to die.

He's significantly less likely to die. He's also much more likely to remain combat effective after taking a hit, which means that even if you end up causing eventual fatal injury to someone wearing this he's still probably going to be able to shoot back.

And while I agree that artillery is going to shred you no matter what armor you're wearing, that's not as effective when your enemy is either dug in (which a peer army could and would do) or irregular (in the case of civil war).

that's not as effective when your enemy is either dug in (

... I don't mean artillery of the kind the Taliban are using - a few rusty 120 mm mortars with a few shells, but the kind actual armies use.

You've heard the stories out of the Ukraine, or seen the pictures, right ? With accurate artillery fire, unless you have a deep shelter several meters under the ground, you're pretty much dead.