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If you search this site for "railgun", you'll find a lot of questions and answers. But none seem to talk about what the ammo wouls be made off.

Considerations:

  • The small-arms have the energy storage and release to do what is necessary.

  • Metals going in excess of 3000m/s (or getting hit by something of that speed) behave like fluids upon impact. https://en.m.wikipedia.org/wiki/Hypervelocity.

  • For my question the small-arms accelerate the projectile to a maximum of 8000m/s on a length of 1m. Longer barrels don't accelerate further but can reduce the recoil.

Considering the above I would like to know: Which materials would the projectiles best be made out of.

What shape should it be.

What kind of damage would it do against various targets (unarmored, armored infantry, armored tanks).

Would it be useful to add kinetic energy by spinning up the projectile in the barrel before firing.

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    $\begingroup$ Material: copper. Shape: long rod with small fletching fins. Damage: similar to a hypervelocity bullet. Useful to spin it? No, you'd just damage the fletching. $\endgroup$ – Ghedipunk Jul 20 '18 at 22:21
  • $\begingroup$ @Ghedipunk Now right that up as an answer! $\endgroup$ – Gryphon Jul 20 '18 at 23:33
  • $\begingroup$ @Gryphon, meh, too lazy right now. Besides, an answer that I'd be happy to upvote would explain why a bolt made out of a ferrous metal wouldn't work, and it's the start of the weekend, not the start of study time. $\endgroup$ – Ghedipunk Jul 21 '18 at 0:04
  • $\begingroup$ @Ghedipunk Just saying, you'd actually get rep off an answer, whereas you don't off a comment... $\endgroup$ – Gryphon Jul 21 '18 at 0:05
  • $\begingroup$ "longer barrels can reduce the recoil" - you mean the peak recoil force? Total recoil must be the same - conservation of momentum. $\endgroup$ – KerrAvon2055 Jul 21 '18 at 0:29
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energy = mv2 where m is mass and v is velocity. A railgun round is a kinetic impactor and you want to maximize energy delivered by your kinetic impactor.

Considering first a railgun projectile in the atmosphere:

Maximize mass. In the atmosphere, the air hits the projectile as fast as the projectile hits the air, and as v increases losses of kinetic energy to air resistance also increase. If there is an atmosphere and any distance over which it will exert friction, it makes sense to maximize m which will not be affected by friction.

Shape. An artillery shell or a bomb delivers an explosive payload. The best shape to deliver maximal volume is a sphere. So a shell deviates from a sphere shape only insomuch as is necessary to stabilize it aerodynamically.

The shape of a kinetic impactor in atmosphere should maximize mass at the same time minimizing the forward profile = minimizing air resistance. This favors a long thin pointed projectile - which is how kinetic weapons look - both real kinetic impactors, railgun projectiles and fictional orbital kinetic "rod from God" type impactors. The smaller the forward profile, the less air resistance there is. Depicted - a real kinetic antitank weapon.

https://en.wikipedia.org/wiki/Kinetic_energy_penetrator kinetic penetrator

Stabilization: fin vs spin.. Fins use push by passing atmosphere to stabilize. Spin uses gyroscopic action to stabilize. Fins work fine for an arrow, or for a bomb falling from a plane. The problem with fins on a bullet (a small kinetic impactor) is that a bullet is pushed by expanding gasses and pressure in the barrel of the gun. The bullet (or shell) should optimally occlude the barrel to optimize push by gases. Fins get in the way of this occlusion so spin is a better answer for a bullet. Also nice is that the barrel itself can impart spin at the same time it confines gas pressure: the barrel does double duty.

I have wondered before why kinetic impactors and railgun projectiles had fins (which necessitates a sabot or external breakaway casing, which you can see in the image). I assumed the sabot was because a shape optimal for being propelled from railgun or cannon was not optimal for the projectile and so there had to be a shape change - this is probably part of it. I think you need a sabot to accommodate the fins. But why not spin the kinetic projectile like a rifle bullet? The fins make a little bit of spin but not like a rifle bullet.

https://en.wikipedia.org/wiki/External_ballistics#Long_range_factors There are issues with spin which can cause "spin drift". The farther your projectile goes the more problematic this becomes. Also, because spin drift has to do with interaction between the side of the projectile and the air, the more side a projectile has the more interaction of this sort occurs - so for a long thin projectile spin drift would be worse. Probably even for spin stabilized projectiles like shells or rifle bullets there is a point where added stability from added spin is counteracted by increased spin drift - so there is a maximum of spin you should have before your payoff decreases.

So: a very massive kinetic projectile for long range use should be long, pointed and have fins. I suspect long range sniper bullets and antitank bullets might be better off with this configuration but I have never seen one (readers - if you have please insert image!). Maybe a sabot is just too fussy for small arms and if you are shooting over super long ranges you should have a mounted gun.


Yet here were are, in a fictional world where cyborgs with onboard aiming software are firing small railguns with long barrels like buffalo rifles. Can we have a projectile without cumbersome fins and sabots? I propose that this small railgun projectile be spun internally. Within the conductive housing (which would be silver, the best conductor) is a massive osmium cylinder. The cylinder is spun up to immense speeds while in the gun using something like a drill. The soldier will need to do the aiming first because once the projectile is up to spin it will be very hard to shift the gun laterally. The internal cylinder does not interact with the atmosphere to cause spin drift and the massive osmium retains its rotational inertia very well. This thing will fly straight, no sabot, no fins. The sharp tip is a diamond.


What will happen with this or any hypervelocity kinetic impactor: if it hits a yielding target it will continue through to the other side and along its path. A shockwave will propagate through air, fluid and solid spaces within the target, disrupting other things within the target. An unyielding target will heat up or break, and pieces of the target will fly in an expanding cone through more distal parts of the target (spalling), disrupting what they encounter; the shockwave described above will also form. A near miss will cause damage via the shockwave through the air; this is true and was described even for cannonballs.

Werewolf infantry will be very surprised. Hopefully they are advancing in a straight line right towards you.


Space: In space there are different considerations, because air resistance is not an issue. As noted here Railguns designs for hand held rifles and spacecraft. You will not lose energy to friction by the air and so there is no need for spin or fins. The shape should be that which is best accelerated by the rails. It is possible you might disregard mass in favor of projectiles which can be accelerated to very great velocities since energy increases with the square of the velocity.

I envision the space railgun, barrel a kilometer long, solar panels charging the capacitors. It then spits out a little superconducting beryllium bead full of pressurized hydrogen, moving at 0.9c.

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    $\begingroup$ Even rails guns in space benefit from spin during the acceleration phase. Not in flight stability, but in averaging out the deviation from the desired path due to imbalances in the thrust of the ammo. $\endgroup$ – Gary Walker Jul 22 '18 at 0:21
  • $\begingroup$ @GaryWalker - I would like to read more about that. Could I trouble you for a link? $\endgroup$ – Willk Jul 22 '18 at 2:55
  • $\begingroup$ Outside of the question, could you perhaps add some info about Creed Archon's Tungsten? It looks like the most density material and considering the maximum speed is attained regardless of projectile weight (unless it's shorter than 1m) it would hold the most kinetic energy. But wouldnt the hardness wear out the railgun (or would a coilgun be better?), and might a different material like gold or more easily attainable lead be more useful despite their softer nature? The "spin a piece inside the bullet for stability" looks like a stroke of genius. But why would the spin not contribute to power? $\endgroup$ – Demigan Jul 22 '18 at 19:21
  • $\begingroup$ Here is a metals density chart. sciencetrends.com/the-density-of-all-metals. Osmium is the densest but I understand it is rare and also really hard to work with. Tungsten is more tractable and so dense projectiles in RL are tungsten or depleted uranium if you can get it. Coilgun vs railgun are so different it is hard to address both with same concepts. Re rotary intertia and power - interesting. I had not considered where the kinetic energy stored as rotation goes when the projectile hits. I have to think it would be a small fraction of the kinetic energy imparted by the rails. $\endgroup$ – Willk Jul 22 '18 at 19:27
  • $\begingroup$ @Willk -- Sorry, no link -- just a degree in mechanical engineering. Thrust is never perfectly balanced, and it is obvious and well-known that a spin axis will tend to balance thrust along the spin axis. Depending upon rail-gun design, this could be a minor or significant improvement in precision and accuracy. $\endgroup$ – Gary Walker Jul 23 '18 at 11:38
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Since the options are; conductive vs magnetic.

Exhibit A Aluminum!

Since the 2 poles extend across rails, the inert, safe & cheap projectile can be from aluminum. More speed since it is lighter. Means you can carry more ammo.

Exhibit B Ferrous- steel

Coils don't touch the projectile. Hence hardness isn't an issue over the barrel. Propel a fletchette of crude iron for deforming on impact. orrrrrrrr Kinetick penetrators of Steel. So you are firing Armor Piercing rounds.

Plus! You can fire delicate rounds with tech or explosives, since there is no heat buildup.

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  • $\begingroup$ don't forget Tungsten, solid and a hard hitter prefect for rail-guns $\endgroup$ – Creed Arcon Jul 21 '18 at 7:28
  • $\begingroup$ Want to take it past customs? Graphite! A pack of graphite pencils will do. $\endgroup$ – Gustavo Jul 21 '18 at 14:18

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