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Magnetically Rifled Smoothbore

air gun.
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(+6, -1)
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The pellet is of iron (nitrided for lubrication and corrosion resistance), or maybe aluminium would work. Either way, not lead.

When firing, the first stage of the trigger pull spins the pellet up to high RPM in the very-slightly-larger ceramic chamber, the second stage does the usual whoosh-it-through-the-barrel thing, by whatever pneumatic machination.

Intrinsically more precise, ergonomically more accurate, environmentally friendly, less energy usage (reduced friction), cheaper (simpler to manufacture a quality barrel), more robust in the long term (no real barrel wear and tear to speak of... the barrel might be made of aluminum even - quite lightweight - with a steel shell to keep it from getting bent out of shape).

On the downside ammo is slightly more expensive and, if you run out of battery power, the pellet won't be rifled, and the effective range tanks.

FlyingToaster, Jul 03 2016

Be the Flywheel Be_20the_20Flywheel
[mitxela, Jul 04 2016]

auto-adjustable rifling Auto-adjusting_20ri...ity_20and_20so_20on
mit zern anti-coriolis effect [not_morrison_rm, Jul 05 2016]

Spinning barrel test rig http://nerfhaven.co...g-rifling-test-rig/
Mentioned in my anno. For Nerf darts, which are ballistically stable. [notexactly, Jul 19 2016]

Spinning tube and magnet Lenz's law demo https://www.youtube...watch?v=CuGgNjkquxc
Proof that Lenz's law-based rotation works with homogeneous rotors (as I expected) [notexactly, Jul 19 2016]

[link]






       <sigh>   

       The spinning projectile is pushed into the smoothbore barrel by gas pressure.   

       The projectile needs to contact the bore to give a gas seal.   

       Even with lubrication, the friction will greatly reduce the spin; worse, said reduction will be inconsistent, throwing the ballistics off.   

       It's likely to be less accurate than a simple smoothbore with a rifled lead projectile.
8th of 7, Jul 04 2016
  

       <tsk>   

       The chamber - not the bore - is slightly wider than the pellet; just enough for an air-bearing effect when it's spun up. Barrel entry would be at a consistent RPM and velocity so, barring small vagaries for temperature expansion, and given consistent ammunition and a non-fouled barrel, ballistics should be fine.   

       While the barrel entry RPM is greater than exit, friction would still be less: no lands/grooves, so not as much surface to frict. And faster: no energy is wasted distorting lead off the lands and into the grooves.
FlyingToaster, Jul 04 2016
  

       [+]   

       When I read the idea I was thinking EM rifled. Think very long induction motor.   

       Has anyone tried spinning up the barrel ?
bigsleep, Jul 04 2016
  

       Or a permanent magnetic field that spirals along the barrel. Probably wouldn't apply much torque.   

       Spinning the barrel however, that's genius on the same level as Be The Flywheel. <link>
mitxela, Jul 04 2016
  

       I imagine that you could put magnets in a spiral along the barrel to make a bullet spin without needing grooves.
Interestingly, metals don't have to be magnetic to be affected by a magnetic field, but the strength of the effect varies with conductivity. It is also dependent on structure. Potentially the bullets could be optimised for the weapon, being wrapped with wire somehow. Without working it through I'm not sure what the best form would be.
  

       Roughly how much energy is lost to distortion and friction in the rifled system, anyhow?
Loris, Jul 04 2016
  

       gah, ninja'd by mitxela. Still an interesting idea, though.
Loris, Jul 04 2016
  

       This concept (spin up then pneumatically inject) is sometimes bandied about for railgun projectiles. Again the projectile must contact the rails and so will lose spin to friction.   

       That is, if the rails are conventional solid conductors...
bungston, Jul 04 2016
  

       Grabbing something with a magnetic field takes awhile: the pre-spin system has a whole tenth of a second to do its stuff (during the first trigger pull stage), as opposed to a thousandth of a second (during barrel travel) for a magnetized barrel spiral.   

       //how much energy is lost to distortion and friction // I"m sure I've no clue. Maybe somebody with decent smoothbore and rifled .177 airguns can give a shot at pushing pellets through.
FlyingToaster, Jul 04 2016
  

       Ok, there's a slight problem here: the projectile won't spin. In order to apply a torque to it, it's going to have to be something other than a "pellet" - it'll be, by definition, an electric motor's rotor. Which, you may have noticed, is not a homogeneous object.   

       Basically, all you have is a normal air-gun with an electric bullet pre- heater. With batteries.   

       //Has anyone tried spinning up the barrel?// Yep. See Gatling derivatives (e.g., Vulcan cannon). If you mean for bullet stabilization - well, say you're trying to match a 1 in 16 inch twist at 750 fps, which is fairly modest; that's over 33,000 rpm. Dental drill speeds for a part which may be 30% of the weight of your gun... if somebody tried it and survived, they'd have a helluva story to tell afterward.
lurch, Jul 04 2016
  

       Like a pellet in a gauss gun, except rotational instead of linear motion. (I imagine) it's the same as an electric motor, except less efficient because there's no active magnetism on the pellet: for aluminum it has to wait until a current is inducted which then produces magnetic force to push against. Or other similar words to that effect.
FlyingToaster, Jul 04 2016
  

       //While the barrel entry RPM is greater than exit, friction would still be less: no lands/grooves, so not as much surface to frict//   

       I highly doubt it. The mass of the projectile is fairly low compared to the friction forces you're going to be dealing with. One option would be rotating bands as per some tank main gun rounds - but not really doable in the sizes you'rew probably dealing with.   

       I can't imagine this being cheaper or more robust than a conventional rifled barrel system. Especially if the power level you're marketting to is comparable to high powered air rifles. They have barrel lives of 10's of 1000's of rounds, generally the firearm is worn out or redundant before barrel replacement anyway.
Custardguts, Jul 04 2016
  

       //Like a pellet in a gauss gun, except rotational instead of linear motion.//   

       OK, try this: take a pellet in your hand, hold it out, and drop it. EXCEPT: have the gravity make it spin instead of fall. That's precisely what you are saying.   

       The way the pellet reacts to your electromagnetic field is the same way it reacts to a gravitational field - a vector only. If you want to get a torque couple, you need to have current flowing one way in one part of the pellet, and the other way in the other part of the pellet. So - the two parts of the pellet need an electrical insulator between them. Then you need to either match the rotation of the field generator to the rotation of the pellet, or make multiple current paths through the pellet which can be switched to keep an appropriate angle to the exterior field.
lurch, Jul 05 2016
  

       Or not. If you've a pellet within a long coil then it will shoot out of the coil, yes ? and if the pellet were outside, but next to the coil, then it would also tend to move in the direction of the coil's axis, yes ? So, if you bent the coil around to make a circle, a pellet inside the circle would spin.
FlyingToaster, Jul 05 2016
  

       I love this idea. It's definitely possible to get enough force between a helical magnetic arrangement in the barrel and a complementary magnetic set up in the projectile, but can you make it practical. As presented I don't think so. But, imagine an artillery piece, you could have electromagnets arranged around the barrel describing the ideal rifling profile. A tremendous amount of current may be necessary to exert enough force, but only for a fraction of a second. That sounds like a job for a big capacitor bank. The projectile could have a piezoelectric current, generated by the large force common in the firing procedure.
bs0u0155, Jul 05 2016
  

       I love piezoelectric. Because I love pie. Even the zoelectric ones, I think.
bungston, Jul 05 2016
  

       // and if the pellet were outside, but next to the coil, then it would also tend to move in the direction of the coil's axis, yes ? // No.
The magnetic field outside the coil (actually, a solenoid, based on your usage) is zero (theoretically) or very small (practically) and is rejected by a conductor placed near the field (honestly).
lurch, Jul 05 2016
  

       // So, if you bent the coil around to make a circle, a pellet inside the circle would spin.//   

       Are you advocating a helical barrel ?   

       //So - the two parts of the pellet need an electrical insulator between them.//   

       Or use a nibblydibnium permanent magnet. But yes, there needs to be some polarity in the projectile*. In some motors a polarity is induced into a lump of conductor so the rotor reacts against the stator windings. So to get the projectile to spin is all about clever electronics along the length of the barrel.   

       *bizarrely you can get a bullet to curve if its uni-polar charged. Any net charge moving through a magnetic field causes a force. So for example, if your barrel strips electrons from the bullet as it fires, the net charge gathered would curve the bullet due to the earths magnetic field.
bigsleep, Jul 05 2016
  

       Like a coil - say a ballpoint pen spring - which is bent'round so the ends (almost) meet. Or something else that gets the pellet to spin.
FlyingToaster, Jul 05 2016
  

       // Or something else that gets the pellet to spin. //   

       If the pellet had a spiral grove cut into its exterior in the same direction that the coil would impart spin then high pressure air would assist the spin.
whatrock, Jul 06 2016
  

       /bizarrely you can get a bullet to curve if its uni-polar charged. Any net charge moving through a magnetic field causes a force. So for example, if your barrel strips electrons from the bullet as it fires, the net charge gathered would curve the bullet due to the earths magnetic field/   

       I can tell when the coffee is working because comments like this turn into SF short stories in my head. This story would involve a space artillery duel in which the combatants use charged bodies to impart charge to and then curve the paths of the incoming projectiles. I remember an SF tale where the docking ship had lightning bolts jumping off of it as it neared the dock. Presumably there must have been some loose gas in the area to create the plasma.   

       I bet items which have been in space for a while can generate really big static electric charges from stray molecules / solar wind etc. There must be some way that the astronauts bleed that off before they go wrestling the Hubble around or they would get a big shock.   

       One can definitely attract floating cat hairs with a rubbed balloon. An interstellar analog of that maneuver should feature in the SF story.   

       Yes, good coffee. Pickled herring too. I will need to try that combo again.
bungston, Jul 06 2016
  

       // Has anyone tried spinning up the barrel ? //   

       Yes*: [link]   

       *For ballistically stable projectiles (for which it is unnecessary), which bullets and pellets are not.   

       // Ok, there's a slight problem here: the projectile won't spin. In order to apply a torque to it, it's going to have to be something other than a "pellet" - it'll be, by definition, an electric motor's rotor. Which, you may have noticed, is not a homogeneous object. //   

       No: [link]
notexactly, Jul 19 2016
  

       Furthermore, for spherical projectiles, don't you usually want to give them backspin rather than roll? That gives more range via the Magnus effect, and rolling for stability is probably not helpful when the wind can't destabilize the projectile in the first place (which, I believe, is the entire reason bullets need to be spin-stabilized).   

       That's what paintball barrels do. I believe it's called a hop- up.
notexactly, Jul 19 2016
  

       // No: [link] // So you found a video with it working on a ring. Cool. Except - it's a ring. That is, it has a hole in the middle, and an electrical path around it, meaning that it can act as a one-turn coil, and it's not a homogeneous conductor. Fill in the hole, and try again. Or make some negotiation with [FlyingToaster] about perforated projectiles which need to work with //whatever pneumatic machination//...
lurch, Jul 19 2016
  
      
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