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I have a setting where some units of specialized infantry use ridiculously over-engineered power armor similar to that worn by the Brotherhood of Steel in Fallout or Space Marines in Warhammer 40k. While the top rating for modern body armor seems to cap out at being able to stop .30-06 rifles, this new class of armor is rated to stop everything up to squad-level heavy machine guns. Because these infantry use robotically assisted exo-suits, they can carry a lot more weight than a normal soldier allowing them much thicker body armor.

The specs I am looking for are the ideal material to make this armor out of, how thick its plates would have to be, and how much the full set of armor would weight.

  • Assume a tech level with a generally current understanding of material science.
  • For purposes of what constitutes a "squad-level heavy machine gun", let's say this armor has to be able to stop a .50BMG-SLAP round fired from an M2 Browning Machine Gun.
  • The armor must offer this level of protection to the chest, back, shoulders, fronts and out-sides of legs, front and outsides of arms, and most of the head. Thinner armor in more articulated areas like the hands is to be expected.
  • Solving the engineering hurdle of the percussive force killing the guy inside without penetrating the armor is not necessary, but would be interesting information to include.
  • Including the weight of the robotically assisted exo-suit needed to carry the armor is also not necessary, but would be interesting information to include.
  • The best answer is going to based on the lightest material you can use that keeps the plates thin enough to be used as described above.
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    $\begingroup$ Since you tagged this hard science, I think that if anybody had this at hand would rather patent and sell it to the highest bidder rather than posting it here under CC license. $\endgroup$
    – L.Dutch
    Dec 31 '20 at 17:18
  • $\begingroup$ @L.Dutch-ReinstateMonica I am not looking for a completely new type of armor. Just existing types of armor that could be applied to this situation. Several sources I've seen over the years say, the only reason heavy power armor is not already in wide-spread use is because of limitations in being able to power it. The power source I expect to have to handwave, but as for the best kind of armor to use, that should be solvable, I'm just not sure which of many different kinds of armor would do the job best. $\endgroup$
    – Nosajimiki
    Dec 31 '20 at 17:37
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    $\begingroup$ I'm confused. Aren't armored personnel carriers (APCs) extremely common in the modern military? What is an APC other than powered armor suits capable of stopping machine gun bullets? They sure cannot stop anything bigger. $\endgroup$
    – AlexP
    Dec 31 '20 at 20:43
  • $\begingroup$ @ AlexP APCs are not necessarily designed around optimising armor weight & thickness the way that you would need to with articulating armor. Many lighter APCs can't survive heavy machine gun fire and many heavier APCs conserve weight by using spaced armor plating which is often > 6" thick (not practical for power armor). 50cal BMG-SLAP rounds are rated to punch through .75" of armor grade solid steel at ranges of 1500m or up 1.5" of steel at closer ranges. So I'm looking for something thin enough to articulate, but lighter than solid steel which would weigh ~1200kg. $\endgroup$
    – Nosajimiki
    Dec 31 '20 at 22:56
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The obvious first step is to look at real-world vehicles which are rated to resist HMGs. STANAG 4569 Level 4, even if that might overestimate things a little (the ex-Soviet 14.5mm vs. the Western .50-cal).

Googling a bit says 16.5mm composite plus 15mm ceramic, at 90.5 kg per square meter.

A human has 1.5-2 square meters skin area. The outer hull of your suit will be larger than that, but how much? Square-cube comes into this, so call it 4 square meters, 360 kg, 800 lbs. of armor.

Let's hope your powered armor can carry that, in addition to the motors and power systems, weapons and sensors, and of course the operator.

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