And if not, how could you reinforce it so that it would? I’m trying to figure out if it’s realistic in any possible way for a cyborg to dodge or deflect a bullet, and while my initial assessment of the situation led me to believe “better not accelerate any of the squishy bits at over a 1000g if we want to keep using them later”, replacing one arm with a fully synthetic one dedicated to the task that would move to deflect the bullet while the body remained still seems like it might be the way to go.

But then we run into the biggest problem with this beloved superhuman feat, kinetic energy. A lightweight prosthesis + some sort of one-handed weapon, be it a baton, bat, sword etc. would probably mass around 3.2kg on the lower end, and in order to deflect most bullets from a reasonable distance, a top speed of 1200 m/s would be ideal for our ninja arm. Unfortunately, most pedestrian weapons would break into pieces at the sudden acceleration up to this ass-cracking speed, and even assuming the same problem doesn’t apply to your arm or any of its more delicate components (which it might), you still have to face the inevitable reality that by swinging this magical bullet-deflecting arm, you’re going to create a shockwave like a grenade going off right next to your body, just without the shrapnel. It’d probably be about equal to 2 megajoules all told, though not all of that would be going into the shockwave.

Can you even begin to modify a human body to survive this? Reinforce the bones with fullerenes and install subdermal polysaccharide armor, maybe? What about organ damage from the shockwave or the arm ripping itself out of the socket? Can you prevent this? Or would it be smarter to have a bullet-deflecting drone you command remotely, or simply modify yourself so getting shot is less of an issue?

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    $\begingroup$ I have to wonder why you think your ninja arm needs to move as fast as the fastest bullets, it is moving a much shorter distance, so it can travel much slower to intercept even the fastest bullets. keep in mind a military rifle bullet is only traveling a little more than half that speed 800m/s. 1200m/s is only achieved by putting a lot of powder behind a very small bullet which only happens in very specialized shells because you end up with worse performance than a large slower moving round. You are also severely overestimating your shockwaves power. $\endgroup$
    – John
    Nov 20, 2017 at 5:24
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    $\begingroup$ Your real issue is newton's third law, if your arm is accelerated up to that speed the body of the person it is attached to experiences an equal but opposite force, which means it is equivalent to getting hit by a high speed bullet the same mass as the arm ( basically a tank shell to the shoulder joint.) if you are very very luck the arm will just rip itself off the person before it can transfer much energy. $\endgroup$
    – John
    Nov 20, 2017 at 5:37
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    $\begingroup$ When the Tzar Bomb exploded (way bigger than your 2 MJ explosion) 100% of humanity survived the shockwave. It was just a matter of being far enough from the explosion place. $\endgroup$
    – L.Dutch
    Nov 20, 2017 at 6:26
  • $\begingroup$ just step aside, at least so that the bullet only flies through non lethal body parts. Much cooler and much more ninja, "be like the water" and such. $\endgroup$
    – Henning M.
    Nov 20, 2017 at 22:17

3 Answers 3


2 MJ isn't too bad. It's about the energy in 2 Snickers bars. The trick, of course, is that it's being released very rapidly. While it may be a reasonable amount of energy, its a very high amount of power.

2 MJ is also roughly the energy of 2 MK3A1 hand grenades (anyone else bothered that a hand grenade and a Snickers bar have roughly the same energy?). So you can use those as a metric. Their official lethal radius is 2m based on their concussive force. That's reasonably small, and easy to work with (they're much more dangerous in closed areas).

If you knew this was coming, you could defend against it. I would rely on an outer shell of very hard material, which helps reflect the energy of the shock wave away from the person. I'd also probably want to have a mask that lets me breathe through baffles to prevent the shockwave from hurting me that way.

Hard to defend against? Of course. It's literally a weapon used by the US military. However, when compared with the challenge of developing cybernetics capable of blocking a bullet with a bat, I don't think it'd be much of a challenge at all.

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    $\begingroup$ "PRIVATE! This is a war zone, not a snack bar!" "But Sarge... the Snickers..." $\endgroup$
    – Samwise
    Nov 20, 2017 at 19:49
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    $\begingroup$ So thats why a Snickers bar has a pull pin..... $\endgroup$
    – Thucydides
    Nov 21, 2017 at 0:02

Mythbusters had an episode where an Iado practitioner deflected a bullet with a katana (can't find it now, they expected a fail and were surprised). I can't find that episode right now, but the real issue was the timing, not the energy of deflection. He had to be an exact distance from the muzzle and in control the firing. The theoretical cyborg would need better reflexes and shorter decision time, since it would not be a staged event.

  • $\begingroup$ Google mythbusters bullet vs sword Top hit feature video is not from Mythbusters show, but some Japanese show, cutting a bullet in half in-flight. MB version is 3rd hit for me: youtu.be/8BJOfsnSA2w $\endgroup$
    – JDługosz
    Nov 20, 2017 at 6:39

So, a bullet travels at 800 m/s. Assuming that your cyborg is deflecting a shot from 20m away, he has 0.025s to react. Your cyborg has highly enhanced reflexes, giving him a lightning-quick reaction time (human average for visual stimulus is 0.25s according to Google, let's say your cyborg is 25 times faster) of 0.01s.

After seeing the shot - he cannot have heard it, because by the time the sound reaches him the bullet will have long passed - your cyborg has 0.015s to move his bullet-deflecting arm into position from what I will assume is a resting position (i.e. arms hanging loosely by his side). The average arm is 70cm long, so bringing his deflector up in an arc to end right in front of his face (approx. 90-degree swing) will require the end of his arm to cover a distance of about 1.1m.

The speed needed to cover this journey in the time allowed is 73 m/s - way, way, WAY less than your estimate. I wonder if my calculations were wrong, but in any case I hope they help. Sorry this answer doesn't actually answer you; it was just too long for a comment.


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