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In Sci Fi, the majority of weapons come in the form of either some kind of energy beam/bolt, or as testosterone-dripping slug-throwers. In Mass Effect and the Aldreaverse utilize weapons that fire small metal shavings that are much, much smaller than today's fire arms.

These projectiles are often depicted creating an ionized trail in their path due to the sheer speeds they travel at (ranging from supersonic, hypersonic, or even flat out a percentage of c).

For now we'll ignore how they are fired (advanced magneto gravitic space magic tech or whatever else the universe uses), and the question of the projectile's actually doing anything to the target instead of sailing right through will be explored in another post.

For now, all I'm asking is such a projectile be feasible in atmosphere?

Some characteristics:

  • The projectile is a sub gram metallic projectile

  • The projectile travels at extreme velocities

  • The projectile is so fast, it leaves behind a ionized corona that is visible to the naked eye

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    $\begingroup$ Obligatory xkcd baseball @ near c. $\endgroup$ May 20 at 7:24
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    $\begingroup$ I don't know the math to prove it, but I highly suspect such a projectile would be vaporised by frictional heating. $\endgroup$
    – ShellGhost
    May 20 at 8:12
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    $\begingroup$ @TurtleTail The logic there was the projectile mass shaved from the ammo block was so small ammo that you had infinite ammo in practice. $\endgroup$
    – DKNguyen
    May 20 at 14:23
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    $\begingroup$ @DKNguyen: Wrex even references an incident where he actually did manage to run out of ammo during an extended mission. $\endgroup$
    – Kevin
    May 20 at 18:00
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    $\begingroup$ IMO a fast moving but tiny projectile like that is unlikely to do significant damage. However, if it's ionizing the air that opens other possibilities. Namely: if you create a large electric charge, it will follow the ionized path directly to the target. This makes a "vaguely" plausible lightning gun $\endgroup$ May 20 at 19:17

3 Answers 3

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The answer is "no," but just how it fails depends on the details.

If the projectile is a "shaving", a curved sheet of material, rather than axisymmetric, then it's going to be aerodynamic enough that it will curve in flight, and you won't be able to hit anything. So we discount that.

If its mass is less than a gram, and it's moving fast enough to ionise the atmosphere it travels through, but at speeds comfortably measurable in kilometres/sec, rather than fractions of c, it's going to lose its kinetic energy quite quickly and be very short-ranged.

The above two forms could possibly work in vacuum, but an atmosphere makes them useless. Higher speeds get more exciting.

If the speed is a significant fraction of the speed of light, you have several problems. The first is that the projectile will be worn away rather rapidly by the atmosphere, resulting in a gigantic explosion, as per xkcd's relativistic baseball what-if.

However, this won't be a problem for the person who fired it, because his gun has just expended a ludicrous amount of energy. Any faintly plausible level of efficiency leaves enough waste heat going into the gun that it has turned into plasma and vaporised the firer. That effect works just fine in vacuum, so guns that "work" that way aren't useful.

You might be able to preserve the appearances of Mass Effect, by having sub-gram bullets fired at sane high velocities, say 5km/sec, and claiming that in a humid atmosphere, the rapid compression and decompression of air as the bullet passes through it leaves a vapour trail. The trail wouldn't glow, and the light bullets would loose speed very rapidly (drag is proportional to the square of velocity), but it may look right.

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    $\begingroup$ @Bartors Without shape information, trying to do drag calculations is impractical. If a sub-gram mass is ionising enough atmosphere to be visible, it's going to run out of kinetic energy quite soon. The energy for ionisation comes from the KE, and the efficiency is going to be poor. $\endgroup$ May 20 at 8:57
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    $\begingroup$ Of course it would "run of energy" soon. But the question is "how soon"? Is it 3m or 300m? $\endgroup$
    – Bartors
    May 20 at 9:03
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    $\begingroup$ You have a strange definition of "this won't be a problem for the person who fired it"... $\endgroup$ May 20 at 18:12
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    $\begingroup$ @Bartors In real life, heavier bullets with similar launch energy to lighter, faster bullets do deliver more energy at long range. For instance, a heavy subsonic .300 BLK round, which achieves about half the muzzle energy of the light, supersonic 5.56 NATO round - with the same amount of explosive and a virtually identical rifle - has more energy than the 5.56 round at ranges of 300 meters or more, despite taking more than twice as long to get there. $\endgroup$
    – g s
    May 20 at 19:08
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    $\begingroup$ Shape, weight, velocity and rotation. I can't do the calculations, but they would be possible. $\endgroup$ May 21 at 9:49
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Sure

Flechette style ammunition works just fine.

The late 1980s weapon in the second link fired a 0.66g metal projectile - though probably not fast enough to make a pretty light show.

Considerations

The round is going to be a tiny metal arrow. The fins will provide stability, since rifling isn't an option. (rifling requires contact between the round and the barrel to generate spin, and at high muzzle speed this will result in too much friction.)

Drag is probably not going to be too much of an issue. The arrow shape is doing to have a much smaller cross-sectional area than a bullet of similar weight, and smaller area means smaller drag force. The effective range can probably be similar to existing rifles.

Accuracy could be an issue at long ranges. Smaller mass on the projectile means things like wind or rain will have larger effects.

Ion Trail

The ion trail is just wasted energy - probably the designers would keep the weapon just below that threshold for stealth / efficiency reasons. But it's your world.

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  • $\begingroup$ The ion trail could be an optional mode for higher damage at shorter ranges perhaps, where the futuretech power supply puts in even more power and the gun uses a higher muzzle velocity. Presumably it would have much worse efficiency statistics in that mode, and again I think increased drag and such would remove most of that advantage at long range, but it might be a viable option. $\endgroup$
    – Ben
    May 21 at 10:13
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The previous answers address the projectile viability, but there's a logical consistency problem, too. Real weapons don't have the firepower they have because it's the "right amount", they have it because it's the best compromise between firepower, ammunition supply, and weight available with modern technology. Drastically improving the energy density of your power source from that of chemical explosives won't just change the amount of ammunition that soldiers carry. It will dramatically change the way that people fight, just like moving from the energy density of muscle power to the energy density of chemical explosives revolutionized warfare in real life.

For example: if space gravity magic has a power output commensurate to a chemical explosive, the energy of hundreds of rounds worth of chemical explosives, and the size of a C battery... that's a hell of a bomb in package small enough to shoot out of a machine gun or load into the breach of a recoilless rifle. Your setting either needs shoulder-fired weapons with the firepower of bombs and artillery pieces in addition to guns that shoot tiny bullets really fast, or it needs a compelling reason why it doesn't have them, despite appearing to have the technology to make them.

If my squad and your squad get in a firefight at normal (half-kilometer or so) real life combat distances, I don't want to shoot thousands of rounds at you while you shoot thousands of rounds at me over the course of the next several minutes and hope that I can gain the advantage with infantry tactics, or that my air strike or artillery support gets here before yours does. I want to win the fight right now with an overwhelming advantage in firepower. Which means I want every guy in my squad to shoot ten of those magic batteries at your position and turn you, your whole squad, your entire firing position, and most of the hill you're standing on into a smoking crater.

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  • $\begingroup$ ...Why do I get the feeling you play paintball by dropping a grenade into a gallon tin of semi-gloss and hurling it at the opposing team? 💣💥🏆😉 $\endgroup$
    – FeRD
    May 20 at 21:19
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    $\begingroup$ @FeRD They actually make paint grenades for that. Not sure how effective they are, mind you, but it is a thing that exists. $\endgroup$
    – Ben
    May 21 at 10:02
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    $\begingroup$ @FeRD He's got a completely valid point. If soldiers can throw boom rather than bullets without ammunition being a problem they will. Soldiers normally use bullets because boom and it's launcher are too heavy to carry many rounds. $\endgroup$ May 23 at 15:21

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