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In the semi-distant future, humans are fighting robots with steel armor tough enough to reliably stop a round fired from a regular infantryman's rifle of the time (say something roughly equivalent to 7.62 NATO).

Someone comes up with what they think is a brilliant idea: "If we can't just punch through their armor, why not let the armor do the punching for us?". This person creates a round the same size as the standard round that operates on the same principles as High Explosive Squash Head (a piece of plastic explosive in a metal shell that pancakes onto the armor and detonates, sending a shockwave through the armor which causes bits of metal on the other side to break away and fly at high velocities through whatever else is there). He tests it against some old wrecks salvaged from some battlefield, and...

The question here is two-fold:

  1. Is it feasible to create a rifle-calibre HESH round for general infantry use and have it fire out of a rifle originally designed for standard ammunition?

  2. Would the round be powerful enough to cause enough spalling to reliably damage the internal components (wires, sensor suites, batteries, etc.)?

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  • $\begingroup$ Welcome to worldbuilding.SE! When you have a moment, plese visit our tour() and help center() pages to learn more about us. We need to adjust your question to be more answerable. Given time and technology, the answer to (1) is always yes. Given enough brittleness of the armor, (2) is always yes. So we need more information to make the question meaningful and to avoid the dread "too broad" and "primarily opinion-based" closure reasons. Newton's 3rd law suggests the bullet mass isn't great enough unless the armor is brittle enough on the interior. Only you can know that. Is it? $\endgroup$ – JBH Apr 5 '18 at 21:08
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    $\begingroup$ Nope. The reason you're getting "you should use this instead..." answeers is because of Newton's 3rd law. For every action... Basically, if the armor is hard enough, then the squashed bullet will simply be blown off the armor because the mass of the robot and impenetrability of the armor is, well... infinitely greater than the mass of the bullet + impact force. Your hope is the shock wave blowing a bit of armor off on the inside, but that will only happen if the armor is brittle. If you hit a chunk of raw iron with a hammer, you can bang a piece off the back. You can't do that with steel. $\endgroup$ – JBH Apr 5 '18 at 21:49
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    $\begingroup$ In a sense, I'm forcing you to answer your own question. Is the armor brittle enough to spall on the inside? It probably isn't, or it's not a great armor. But, you must decide as you've told us nothing at all about the physics of the armor. If it's brittle enough, yes and yes. If it's not, yes and no. Either way, question answered. $\endgroup$ – JBH Apr 5 '18 at 21:51
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    $\begingroup$ @Demigan, I don't believe it must spall through, but it should spall. The OP didn't tell us anything about the construction of the robot, so we don't know if anything critical is near the surface, if it uses gears, hydraulics, or biomechanical muscles, etc. Most battlebot volume would likely be motor & control, power, and ammunition: all critical systems. My guess is that it wouldn't need to be massive, but it would need the force to tear through circuit boards within 6-8 inches of the spall source. $\endgroup$ – JBH Apr 5 '18 at 23:15
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    $\begingroup$ @JBH I was thinking more from my super-soldier idea. Even with a near indestructible armor a shockwave like from an HESH explosion would spall nothing, but as the shockwave travels through lung tissue, brains and arteries it can rupture them and damage individual cells. Now apply the same to criquitboard, some battery's/capacitors, more vulnerable moving parts etc. Even if it doesn't damage them, the potential for displacement of bits and pieces can be catastrophic for the component in question. $\endgroup$ – Demigan Apr 6 '18 at 7:11
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HESH requires a fairly large mount of explosive, but more importantly a fairly low velocity in order to allow the explosive filler to "cake" on the armour before the base fuse detonates it.

enter image description here

How different types of ammunition work

Rifle calibre rounds are not capable of carrying a big high explosive charge, and are generally fired at high velocity so the shooter has a flat trajectory for aiming.

More modern HEAT and APDS-FS rounds are fired at very high velocities to achieve a flat trajectory (and easier for gunners to aim), and in the case of APDS-FS to generate the kinetic energy necessary to punch through the armour. HEAT rounds convert their chemical energy into a hypervelocity jet in a period measured in milliseconds when a point fuse strikes the armour, so lower velocity really does not change the mechanism of the round.

In order to get a reasonable round that a rifleman could use, go for one of the common 40mm rifle grenades (either fired from the muzzle, or an underslung rifle grenade launcher), which would carry a sufficient explosive charge to make a useful HEAT or a HESH round.

enter image description here

Some types of rifle grenades

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    $\begingroup$ C.f. SLAP ammunition, which is available (currently) in 7.62x51 (as well as .50 BMG). $\endgroup$ – Adrien Apr 6 '18 at 19:48
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    $\begingroup$ @Amusable - I assumed "a round fired from" meant that it was a round in the chamber. Rifle grenades do not "fire out of a rifle", they are launched, after you attach one and place the butt of the rifle on the ground. And depending on the type, you also have to chamber a blank round. I highly doubt this what you envisioned combat to look like... $\endgroup$ – Mazura Apr 7 '18 at 0:37
  • $\begingroup$ @Mazura It's certainly feasible that in the semi-distant future, when armored, killer robots are roaming the Earth, someone will come up with a way to put an effective explosive in a small-caliber bullet, and/or that typical firearms will use larger calibers to you know, be effective against the armored, killer robots. If we suddenly needed to, for some reason, we could start replacing normal bullets with SLAP and maybe even frag12 and HEAT-type rounds. We just don't really have a need - what we use now works pretty well against the squishy primates we fight. $\endgroup$ – HopelessN00b Apr 7 '18 at 3:54
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    $\begingroup$ @Mazura You're right, it's not. I'm not marking this as correct to say "I'm using this idea", it's more like "this post is the one that convinced me that my idea isn't gonna work as I originally planned it." $\endgroup$ – Amusable Bman Apr 23 '18 at 18:49
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Tech Already Exists (Raytheon is basically already into SCIFI level stuff)

This is the pike Missile by Raytheon (still in final testing, soon to be deployed.)

enter image description here

It is fired from a standard infantry 40mm grenade launcher. It is laser guided with a maximum range of 2500 meters, or 2.5 kilometers. It's warhead is an HEDP (High Explosive Dual Purpose) which is to say that it is a HEAT conical shaped charge that also has been optimized to throw shrapnel giving it dual use against anti personnel. As a general rule of thumb HEDP can penetrate twice as much armor as its diameter, giving this missile an optimum penetration ability of up to 80mm of steel armor (a wee bit more than 3 inches).

Its basically the smallest possible guided missile with any sort of real performance. Smaller and you don't have enough warhead to take advantage of the Munroe effect (shaped charge armor penetration physics). Also any smaller and you cant utilize a big enough rocket motor to have any real range. Regular 40mm HEDP grenade launcher rounds would work too but they are difficult to target as they have a ballistic arc and you have to get within 400 meters to have any chance of a hit. In modern combat 400 meters is basically point blank, even regular infantry weapons like the 240 bravo 7.62x51 medium machinegun can accurately engage infantry at 2000 meters. So yeah, I think the PIKE or something like it is exactly what you are looking for.

Regular 40mm HEDP pictured here: enter image description here

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Is it feasible to create a rifle-calibre HESH round for general infantry use and have it fire out of a rifle originally designed for standard ammunition?

Maybe in a .50BMG, but 5.56mm is really narrow.

Could the spalling from this rifle-calibre HESH round reliably damage the internal components of an armored robot?

It would, but defeating HESH is as simple as a thin lining of Kevlar. That's why the only people still using HESH are the Brits and that's because they don't want to pay for smooth bore barrels and HEAT rounds like everyone else.

https://en.wikipedia.org/wiki/High-explosive_squash_head

Since the 1970s, HESH ammunition has increasingly fallen out of favour as armour designs have trended towards layered composites of hard metal and heat-resistant materials. This type of armour is a poor conductor of shock waves. Anti-spalling devices (spall liners), made of materials such as Kevlar, are commonly fitted to the interior surface of modern armoured vehicles to mitigate spalling effects.

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  • $\begingroup$ And all it takes to defeat HEAT is a grille around the outside of the target. Pros and cons to different technologies. Note: by talking about armoured vehicles in your quote above, it refers to tank warfare. Different targets are more or less affected than others. $\endgroup$ – Baldrickk Apr 6 '18 at 10:14
  • $\begingroup$ @Baldrickk so... no spall liners in Bradleys and BMPs? I'm skeptical. $\endgroup$ – RonJohn Apr 6 '18 at 11:11
  • $\begingroup$ I was thinking more of older Russian Blok vehicles, and things like technicals. Especially the Latter, you want to be firing something like HESH at them instead of HEAT or APDSFS rounds. Both of those will punch through doing (relatively) little damage (vehicle will probably still be a write-off regardless) but they will carry on going. Same with shooting in urban environments. HESH is better for that situation. As I said, different weapons suited for different targets. $\endgroup$ – Baldrickk Apr 6 '18 at 21:36
  • $\begingroup$ The Pike missile mentioned by TCAT117 seems interesting. As do RPG-7s... $\endgroup$ – RonJohn Apr 6 '18 at 23:29
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  1. Probably. HEIAP rounds are currently made in .50 caliber and are used in anti-materiel roles to penetrate light armor vehicles (helicopters, APCs, etc). Scaling it down to a 7.62 would give it less power, but it really depends on how armored the robots are.

  2. Yes. Spalling creates small fragments at high speeds, which is sure to damage delicate electronics. Since the spall is metal, it might even short circuit components, even if it didn't damage them. However, the easiest way to counter that would be to apply an anti-spalling coating on the inside of the armor, which is cheap insurance your enemy will or will already have invested in.

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    $\begingroup$ As an alternative to anti-spalling coating, fill any empty space with a lightweight polymer or plastic - not that there should be too much, that would be an inefficient waste of space. The shockwave will then mostly just travel all the way through the robot, and any spalling would occur from the outside armour on the opposite side. It works on tanks because you need a hollow space for the squishy humans to sit in. $\endgroup$ – Chronocidal Apr 6 '18 at 8:05
  • $\begingroup$ @Chronocidal: Actually, I could see the interior of a robot routinely filled with some kind of foam or gel to provide shock resistance and electrical insulation in case of intrusion. Final step during assembly (or following a field repair) would be to spray some kind of expanding closed-cell foam into any empty cavities, then screwing/bonding the access panels back on. As long as it didn't stick to the interior components, it would be easy enough to rip out when access was required. $\endgroup$ – TMN Apr 6 '18 at 14:10
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I still don't think that HESH would be necessary. Assuming the robot armor was strong enough to stop 7.62 AP rounds which generally pens 7mm @ 300m. A 7.62 HESH round isn't going to do anything to that either because it's just too small to generate enough force to damage anything behind the armor. So moving up to your 50 cal (12.7x99mm) round you now have AP rounds that will pen 19mm @ 500m which should be more than enough to get through the armor. If not, a 12.7mm HESH probably still wouldn't have the kinetic energy needed to actually damage the armor.

I think that is the basic problem here... your ability to penetrate the armor with AP shells will always be at a lower caliber than with HESH. Think of it like a medieval suit of armor, would you rather attack it with a dagger or a hammer of the same size? Obviously, the dagger would be more effective because all of the force is delivered at one point. You could use a hammer but you'd want it to be MUCH larger than a dagger so it can deliver a force large enough to damage whatever the armor is protecting. In much the same way, HESH rounds are only made in large calibers specifically because they need to deliver a large force to be effective.

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  • $\begingroup$ ".50, Saboted light armor penetrator, M903 ... can penetrate 19 mm of steel armor at 1,500 yards (1,400 m)." --- your ability to penetrate the armor with AP shells will always be at a lower caliber than with HESH. Do you have a citation for that? It's too bad there's no penetration data for the HEIAP rounds. I also wonder what a M903 round penetrates point blank. $\endgroup$ – Mazura Apr 7 '18 at 0:59
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    $\begingroup$ Think of it like a medieval suit of armor, would you rather attack it with a dagger or a hammer of the same size? Obviously, the dagger would be more effective because all of the force is delivered at one point. Actually, you'd rather use the hammer. Plate metal is good at stopping sharp-force impact, but bad at stopping blunt force impacts. That's why maces and war hammers and the like exist. If you can't get through the armor, you hit it with a hard, blunt object and do blunt-force trauma. Bruise his kidneys, break his arms, etc - make his armor work against him, being dented into him. $\endgroup$ – HopelessN00b Apr 7 '18 at 3:59

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