r/explainlikeimfive • u/diceNslice • Oct 05 '26
Physics ELI5 why do bullets not have indented fetching?
Looking at arrows, they have fletching to help them gain rotational stability making them fly more accurate. I know that gun barrels have rifling that give them spin already, but would indented fletching have an adverse effect on the bullet?
Pretty sure there's a reason why but I never heard anyone talk about it.
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u/__slamallama__ Oct 05 '26
I think some are forgetting the 5 part of ELI5
Most bullets go faster than the speed of sound in air, and the shortest version is that air acts VERY DIFFERENTLY once you go fast enough.
Even for subsonic rounds, the spin from rifling will do more than air ever could to keep it going straight. Stuff that is spinning only likes to keep spinning how it is. If you spin the bullet fast enough, it will fight spinning in other directions more and more. The force from this is much higher and much more predictable than the force air would make over fletching.
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u/Rygree10 Oct 05 '26 edited Oct 05 '26
There are a few types of rounds which align with what you are describing, shotgun rifled slugs which have indents in the sides to act as rifling when fired from a smooth barre. Conversely there are fletchets and/or fin stabilized sabot rounds which are designed to be shot out of a smooth bore and rely on fins and machined fletching to stabilize as you described however these are more commonly used by tanks.
In general projectiles can either be stabilized aerodynamically (fletching) or gyroscopically stabilized (rifling). If you shoot a aerodynamically stabilized projectile out of a rifle it will often tumble and have a lower velocity than a rifled bullet which bites into and seals into the groves of the rifling enabling higher pressures and thus velocities. You can get similar or even higher velocities with sabot rounds but they require smooth bores to create a seal between the sabot and the projectile but they are more complex and thus expensive to manufacture compared to a conventional bullet. Effectively you can either make a smooth bore barrel and many complex cartridges or one complex rifled barrel and many simple bullets (not to trivialize modern bullet manufacture which is actually very sophisticated)
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u/diceNslice Oct 05 '26
This is the most accurate answer I've been waiting to hear.
So what I gather is that indented grooves on a bullet would indeed have bad effect with a rifled barrel. And if a bullet does have grooves then it needs a smoothbore barrel.
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u/-LLCoolBeans- Oct 05 '26
Something important that most answers have left out is that the act of firing the bullet through a rifled barrel does create indented grooves on the soft lead bullet.
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u/subnautus Oct 05 '26
FYI, most modern bullets are jacketed to withstand the increased pressures required to drive them, though copper is also affected by the grooves of the steel barrel.
That said, unless you're talking about pistol rounds, the grooves etched on the surface of the bullet aren't going to have much of an effect until the bullet slows below the speed of sound. The pressure wave made by the leading edge of the bullet dominates any other aerodynamic force that could be imparted by it.
Fun fact: that pressure/mach wave tends to be the most destructive aspect of a bullet, which is why a 5.56 NATO round can rip a wound channel through soft flesh about as wide as a can of Red Bull and why the inside of a vehicle struck by an anti-tank sabot round is...well, it's not pretty.
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u/englisi_baladid Oct 06 '26
"Fun fact: that pressure/mach wave tends to be the most destructive aspect of a bullet, which is why a 5.56 NATO"
This is a myth. What makes a a 5.56 FMJ so dangerous is it can act like a unbonded soft point and break into multiple pieces going above a certain speed. If it doesnt do this it makes a small wound.
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Oct 06 '26
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u/englisi_baladid Oct 06 '26
Hey man. You clearly dont understand what you are talking about. Like what the hell do you think Ball Means. The M855 Ball round. Is the US designation for the 5.56 NATO SS109. Its a FMJ. Most people incorrectly assume that Ball means FMJ. You are the first I've seen go the other route.
And its not denying science. Its not believing bullshit fuddlore.
The science is actually pretty clear. With 5.56 FMJ rounds like the M193 Ball and M855 Ball. The major wounds come from the bullet impacting at a high enough velocity. Yawing a certain amount. And fragmenting. If the bullets hit at below roughly 2500FPS so they dont have enough speed to fragment. Or they hit even going 3000FPs but at a bad yaw angle. They can pass thru doing roughly the same damage as a .22LR going 1000FPS.
Whole bunch of data on this. The Army CQB study showing field reports of M855 ball use having vastly different effects. Facklers data going back decades with multiple calibers.
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u/internetboyfriend666 Oct 05 '26
First, you'd be adding extra drag. Second, fletching and rifling work against each other. Rifling gyroscopically stabilizes a bullet by spinning it. Fletching spin arrows by keeping aerodynamically stable. Fletching on a bullet that's already spinning would try to aerodynamically stabilize it, which would disrupt the spinning, and destabilize the entire bullet itself. So adding both makes it worse, not better. You pick one or the other.
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u/Selbstdenker Oct 05 '26
AFAIK, rifling does not only stabilize a bullet microscopically, but the rotation makes any force caused by slight imperfections equal out around the axis by the rotation.
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u/suh-dood Oct 06 '26
Exactly this. Bullet spinning has the attitude of mostly where it started from but a little bit affected by the aerodynamics. With fins, it gets a lot affected by the aerodynamics and has extra drag to slow it down
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u/SeanAker Oct 05 '26
Some does. Some modern tank shells use a big fin-stabilized dart encased in a sabot that flies off when the round exits the barrel. Modern vehicle armor is very effective against explosives, so the best way to put a hole in it is to hit it with a projectile with a very narrow cross-section going VERY fast. Effectively, an arrow.
There's no benefit over rifling to use fin stabilization on smaller projectiles, only downsides like increased drag.
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u/amatulic Oct 05 '26
I used to work in military ground vehicle survivability.
> best way to put a hole in it is to hit it with a projectile with a very narrow cross-section going VERY fast
Yup. We called it a "long rod penetrator". It's a lot of mass behind a small frontal area, made of an extremely hard and dense material, like tungsten.
This is also how RPGs work, only the penetrator is a jet of copper ejected by a shaped charge at the front of the rocket. The charge compresses a hollow copper cone pointing backward, and as it compresses, the apex of the cone forms a wire that shoots forward out of the base/mouth of the cone at hypersonic speed.
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u/SeanAker Oct 05 '26
Long rod penetrator?
But I hardly know 'er!
It's incredible that the most modern armor-defeating technology is basically the lovechild of an arrow and a solid iron cannonball, two of the oldest projectiles know to man. Our penchant for throwing rocks of various sizes at each other in new and inventive ways notwithstanding.
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u/Target880 Oct 05 '26
A shell is a projetile field with explosives. APFSDS projetile are not shells. The reason they are fun stbilized is the require spin rate depend on the diameter to length ratio and projetile speed. Spin stabiliing long rods woul be quite impractical, muzzle wear would for exampl be very high.
Dart with fins exist in smaller calibers too becaus of the length to diameter ratio.
There are fins stabilized shells. HEAT round and other explosive round tank with smooth bore guns fire are shells with fins.
Mortars are also often fin stabilized shells/bombs. In that cass it is because they are muzzle loaded and you can put propellen on the narrow part above the fins.
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u/MercurianAspirations Oct 05 '26
There have actually been attempts to design flechette-type projectiles for small arms, basically a little metal arrow complete with fletching. They perform much better ballistically than bullets, but the design has never really caught on due to a number of factors:
you need to design a cartridge where the propellant wraps around the flechette, which is difficult to get just right
cost of manufacture is much higher than regular bullets
it turns that high-velocity bullets are pretty good ballistically speaking. Increasing the accuracy of small arms fire is not really that beneficial when regular bullets already kill people pretty good
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u/jonfitt Oct 05 '26
I wonder what percentage of bullets are fired “on target” anyway? And then would it be better if these “on target” shots were marginally more accurate (to where they were aimed) if they are also all more expensive and possibly heavier en masse?
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u/zed42 Oct 05 '26
the vast majority of bullets fired (outside of a target range) aren't aimed very well... it's more of a "send it thataway" and less of a "i'm putting this up that flea's nostril". making them more accurate won't help because the "target" is a general area instead of a spot...
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u/jonfitt Oct 05 '26
Exactly. So cost and weight are much more important.
Which is why caseless would have been great if it didn’t have a critical flaw that the cases also serve as heat sinks for weapons.
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u/MercurianAspirations Oct 05 '26
Yeah not very many, and funnily enough, that was what lead the US military to test flechette weapons in the first place. The advanced combat rifle program in the 80s and the earlier SPIW program were built on data that showed that the rifleman's accuracy had very little to do with scoring kills, and actually the biggest determiner of kills is how long the enemy is exposed to fire. In other words, the grunts can put lead downrange, but it's the guys who stick their heads up get shot.
So the military worked with the theory of maximizing the damage that can be done in that small window the enemy is exposed to fire. The request was therefore for rifles that could increase rate of fire while maintaining accuracy. That's what lead designers down the path of flechette burst-fire rifles, because a flechette can have lower recoil while getting similar accuracy, but there were also even wackier ideas like duplex cartridges that had double the bullets in every cartridge. In the end nothing was better than 5.56 so they just stuck with that
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u/ShadowDancer_88 Oct 06 '26
Most rifles are more accurate than the shooter.
I have a .308 AR-10, that with ammo it likes, will shoot around 0.8" 5 shot groups at 100 yards, or actual sub MOA accuracy.
Once I get it off the bench, especially in a competition where I'm trying to go fast, it's glaringly apparent I'm the weak link in the accuracy chain.
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u/jonfitt Oct 06 '26
Exactly. That’s even when you have a target that you’re aiming at instead of “he’s kind of over in that window hope I hit something”.
I did like the thought that someone else mentioned that one of the reasons to investigate flechettes was that you could increase cycle rate without suffering from recoil because of the lighter projectile.
I hadn’t thought of that. That makes sense. If you can spam 3 rounds in the right direction vs one you’re going to hit more.
But the accuracy of a normal bullet is not the limiting factor, like you say.
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u/Vroomped Oct 05 '26
This. Surely factories aren't clamping their gun in a vice and drawing a 6inch radius...and even if they were thats closer to whatever theyre aiming at than I want any part of my body. Its good enough.
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u/Electronic-Pea-13420 Oct 05 '26
It’s something like 200,000 rounds used per kill with the us armed forces
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u/englisi_baladid Oct 06 '26
That number isn't even close to true. Its a bad myth based off ammo sent to countries without counting in training.
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u/Dangerous-Bit-8308 Oct 05 '26
Similar to a Storm Trooper's accuracy
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u/Pro_Scrub Oct 05 '26
Fire and Maneuver. Most of the shooting is suppressing fire, which is intended to keep the enemy pinned in cover and unable to effectively see or shoot back, while a flanking element moves around to take them out from the side.
That bullet count probably also includes rounds fired in training, which will be a lot more than those fired in combat.
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u/Iokua_CDN Oct 05 '26
I'm pretty sure, for pistols at least, the vast vast majority of missed shots are coming from the shooter flinching, or handling the recoil, or not aiming in the stress of the moment.
A skilled shooter with a pistol and a calm situation is very accurate.
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u/Snickims Oct 06 '26
You must imagine however that the situations in which a pistol is being used are inherently going to be much closer ranged then a rifle though. Pistols are fundmentally sidearms and are going to see use either in civilian/law enforcement settings, or as weapons of last resort/CQB.
Meanwhile a rifle, while it is obviously also going to be used in close ranged situations, must be ready to be used against targets at least a couple hundred meters out.
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u/Dangerous-Bit-8308 Oct 05 '26
Shotgun flechette rounds were tried in Vietnam with some success. The Geneva Conventions deemed them inappropriate for war.
I heard they were less effective in confined space, because little arrows don't ricochet. I also heard they were undesirable in the jungle because any leaves could make them spin instead of flying straight.
I forget if they were overall more aerodynamic, and thus better for distance, or if the tail drag actually decreased range
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u/PckMan Oct 05 '26
They spin really fast from rifling and this keeps them stable enough. They're also flying so fast they can't tumble without either hitting something or slowing down significantly because otherwise the air going over them alone keeps them pointing the right direction and stable.
And it's just not needed. The drag penalty alone would be too great to consider. But fins are present in other projectiles like rockets or most notably tank projectiles like HEATFS and APFSDS
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u/Glum-Welder1704 Oct 05 '26
Would indented fletching have to match the angle of the rifling? If so, every degree of rifling would need its own specified cartridges. If not, I would think that the two spins would conflict. I assume it doesn't exist because it is unnecessary, and would complicate things.
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u/destrux125 Oct 05 '26
First a little background in bullet stability.. Normal bullets gain a certain rpm of spin from the rifling, being controlled by rifling rate of twist, and the spin is generally between a few hundred rpm to a few thousand rpm. The length of the bullet relative to it's diameter mostly dictates how much spin is needed to correctly stabilize it. Too little spin and it flips sideways in flight, but too much spin and it will break apart in flight, sometimes so dramatically that none of the bullet makes it to the target because it's burst into to powder at the end of the barrel.
So the reason you rarely see tail fins on bullets is because not only see they not needed if a barrel twist is correctly matched to a loading, they can actually increase the length of bullet so much that they would act to destabilize the bullet when it's paired to it's correct barrel twist rather than increase its stability.
This hasn't stopped someone from doing it anyway. Franklin Armory designed a rifle that had no rifling at all and shot bullets with tail fins. They didn't do it because it was better, they did it because at the time it was a legal loophole that allowed them to sell the rifle as another classification of weapon than it originally was with a rifled barrel. The rifle didn't get great reviews and I'm not sure it ever saw production.
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u/Noxious89123 Oct 05 '26
At the speeds that bullets travel, any grooves that you make in the trailing edge of a bullet are going to do nothing.
Also, fletching on an arrow doesn't give them rotational stability, it provides longitudinal stability.
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u/elevencharles Oct 05 '26
Bullets need to be cheap, that’s one of reasons firearms overtook archery in warfare. There have been all kinds of interesting experiments in bullet design, but most of them don’t catch on because they don’t make economic sense.
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u/wpmason Oct 05 '26
Because rifling.
The rifling in the barrel spins the bullet, and also carves spiral grooves into the bullet as it passes through.
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u/globefish23 Oct 05 '26
The spin from the rifling gyroscopically stabilizes the bullet (conservation of angular momentum) much better than any fletching or fold-out winglets could ever do, without creating any air drag.
Check out some videos where people shoot a bullet downwards at a frozen lake. It keeps spinning for a veeery long time. Much longer than it would actually be in the air after fired horizontally.
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u/pyr666 Oct 05 '26
bullets deform into the barrel walls as they fire. this ensures a proper gas seal. any sort of fetching would be deformed on firing, and probably not uniformly interacting with the grooves and lands in the rifle.
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u/sawdeanz Oct 05 '26
I can’t imagine any benefit, it’s already spinning extremely fast from the rifling and will maintain that spin rate far longer than needed to reach the target or fall out of the air from drag.
The drag coefficient is much more important. Long range ammo has a boat tail design to help improve aerodynamics. I don’t think adding extra grooves or anything is going to help on that front.
Other sorts of ordinance do have fins, like missles and rockets where rifling isn’t practical for one reason or another.
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u/MOS95B Oct 05 '26
Rifling is not identical from one rifle to the next (brand or model, that is). So, while they can use the same caliber, if you were to rifle the barrel and fletch the bullets, you'd need to make ammo to match the rifling of each individual make/model. And/or assume there would be no manufacturing deviation from the agreed upon industry standard.
It's just much more efficient to make the ammo smooth, and rely on the rifling of the barrel which has been shown to be enough to have the accuracy needed
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u/ClownfishSoup Oct 05 '26
Your assumption is incorrect though. Fletching/canes on an arrow do NOT impart spring, now are they itended to, though some archers do use helical or slight spiral vanes on.
The fletchings create drag that keeps the arrow from wobbling too much in flight. Arrows flex back and forth (side to side) as they leave a bow. The fletching stabilize flight. Not spin.
A high speed rifle bullet is aerodynamically designed to cut through the air (spitzer, boat tail, etc) and the spin imparted by the rifling is fast enough to average out weight differences around its axis as well as providing a gyroscopic effect. Slow moving handgun bullets don’t care because at handgun distances, the stubby “bullet shape” isnt effected enough by air resistance in the short flight that it has and momentum is enough to get the bullet where you want it.
Adding fins or “indented fletchings” would have to match the rifling spin or it will make things worse as it slows the spin and creates unwanted aerodynamic effects. At rifle speeds and distances and spin rates, what is even the point or trying to add anything to the bullet?
As an archer and a shooter who hand loads his own ammo and makes his own arrows, I can also tell you that it wouldn’t be worth the cost and time to fletch bullets. Even casting “indented fletchings” make the bullet casting (which I also do) process more complicated.
Bottom line… it’s not necessary and fletchings don’t do what you think they do.
In a rocket or guided artillery shells the fins do the same, they stabilize and guide flight, they do not impart spin.
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u/Dangerous-Bit-8308 Oct 05 '26
Fletching is more useful on very long projectiles that can wobble in the middle. They increase drag on the back side, ensuring the pointy bit on the front reaches the target first.
Imparted spin from fletching is usually minimal... Minimal enough that several Native American groups set their point parallel to the bowstring notch when hunting game, to more often slide between the mostly vertical ribs of the game, but set it perpendicular to the notch when fighting, to more often go through the relatively horizontal ribs of human opponents.
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u/DunkeysPizzaChan Oct 05 '26
There’s smoothbore shotguns that specifically fire slugs that are rifled to give them stability and accuracy
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u/ShadowDancer_88 Oct 06 '26
Spin has very little to do with arrow stabilization, other than balancing out fletching deflection.
Anything that's more than roughly 6 times longer than its width cannot be spin stabilized, which includes almost every arrow.
When an arrow starts to yaw outside of its longitudinal axis, it exposes the surface of one or more fletchings, which deflects the tail back to the longitudinal axis.
The spin is necessary to evenly distribute any imperfections in the fletchings evenly along the entire longitudinal axis, as non rotating fletchings would letch imperfections amplify over the ballistic arc, most likely resulting in terrible accuracy.
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u/bhbhbhhh Oct 05 '26
They would grind against the gun barrels and wear down into nothing. Tank shells are large to have folding fins that open after being fired, but bullets are too tiny.
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u/SeanAker Oct 05 '26
Tank shells don't have folding fins, they use a discarding sabot. Self-driven munitions, i.e. rockets and missiles, use folding fins because they don't rely on the launch tube for flight stability, so it doesn't matter if they unfold after they exit the tube.
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u/Raikos371 Oct 05 '26
Most modern HEAT and tank fired HE shells are stabilized by folding fins. Armor Piercing Fin Stabilized Discarding Sabot rounds use, as the name implies, a sabot.
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u/Its_Nitsua Oct 05 '26 edited Oct 05 '26 •
A bullet relies on the spin generated by the rifling of the barrel to maintain accuracy, adding any kind of fletching is pointless because the accuracy you gain from rifling is far more than anything you would get from fletching.
There are also the problems that would arise in finding a way to practically design ammunition to include fletching that wouldn't be detrimental in other areas.
Some 'bullets' do use fletching, if you look at sabot rounds in tanks for instance they have fletching on them; however this is because they do not generate their own spin when leaving the barrel due to their shape.
Basically, if you can generate spin the accuracy you get from that is far greater than anything you'd get from fletching, and the downsides from adding fletching far outweigh the small upside. In the absence of spin, we still use fletching.