Why are bullets bullet shaped?

Factual Questions
1

Bullets all tend to have the same profile, like a rectangle capped with a parabola. The trailing end is always flat or concave. I assume this would create a lot of undesirable drag. I always sort of figured that bullets were this shape so that the propellant gases had something to push against when the cartridge is fired. But the other day I was musing (as one does) and it occurred to me that the seal in modern rifle barrels is pretty damn tight, so it wouldn’t matter what shape the bullet was, the gas would still be pushing against an area equivalent to the area of the bore.

So why do bullets have a flat trailing end? Why not make them parabolic at both ends or other aerodynamic shapes?

And I realise some bullets have a slightly tapered tail, but all the ones I know of are at the very least truncated if not outright concave at the very end.

2

Just a WAG here, but maybe it’s because a bullet that was tapered at the back end would allow the gasses to escape the barrel a fraction of a second earlier than one that was not tapered, robbing the bullet of the straightening effect of part of the barrel. IAW, it makes the barrel effectively shorter to have a tapered bullet.

My second guess is the same sort of cultural lazyness that brings us the qwerty keyboard many years after it has no longer been neccesary. Simply, that’s the way it’s always been so why bother to change it?

3

Perhaps its the same reason arrows have feathers. Do I need to go further.

4

It’s so simply - bullets are shaped like bullets so we’ll recognize them.

Besides, they vary quite a bit. Some are as rounded as half-inflated beach balls while others look like miniature Saturn rockets.

5

Arrows have feathers to make them stable - to put the center of aerodynamic pressure behind the center of mass. I don’t think a flat rear end of a bullet has the same effect.

I suspect the answer has something to do with the way the cartridge is manufactured, but I have no idea. I do note, however, that trucks and minivans tend to have the same shape - round nose, flat rear end.

6

Some speculation;

For any accuracy, you would need at least part of the bullet to have a diameter that snugly fits the bore of the barrel. In flight, I’d assume most of the drag would come from the thickest part of the bullet displacing the air (kind of like the leading sprinter tiring himself out, while the sprinters behind follow effortlessly in the wake). If the energy has been spent and the air displaced already, would it make that much of a difference drag-wise if the back of the projectile was equal or smaller in diameter?

Another thing to consider is the mass. If you trim away at the bullet, you’ll make it lighter, and unless you compensate by upping the velocity, your going to be hitting your target with less force. You could speed things up with more powder in the cartridge, but then, why even trim away at the bullet if there is no appreciable difference in drag.

I’m sure a scientist will be along soon to correct, clarify.

7

Vans have a flat back so that you have the maximum usable floor space. A van with a ceiling 6 inches above the floor wouldn’t take much of a load. I can’t see this being a problem with bullets.

Thaumaturge would that be a big problem? If bullet made a barrel effectively 1/2 inch shorter that would seem like a small price to pay for what I guess woould be a more aerodynamic projectile. Considering that artillery rounds have the same shape I’m guesssing there’s more to it.

** j.c. ** don’t both rockets and hemispheres have a flat trailing end anyway? Wouldn’t a full sphere or double pointd rocket be more aeroynamic? I can see why rockets need to have a flat end where the engines are, but bullets don’t carry their own fuel.

8

Blake - All bullets are flat on the end (so they sit snugly, perhaps?) but they don’t really look much alike outside of that. No more than chess pieces look alike.

9

DemonSpawn52, IANAphysicist, but their is a good reason why birds, fish, sports cars and so forth don’t have flat trailing ends. The trubulence created by a flat end creates drag. A tapered end produces a more even airflow. The cross sectional width is important as you say, but I think that doesn’t mean one can’t have a taper no matter what the cross section.

I’m also not sure that trimming would be the only solution. Couldn’t one simply add more material to the tail, increasing the weight of the bullet?

10

Several reasons.

First, having a blunt rear end gives you more room to pack gun powder into the cartridge. The extra propulsion more than makes up for the slight increase in drag caused by the turbulence trailing the bullet.

Second, if a bullet were shaped like a football, then the slight, but still existent, gap between the bullet and the inside of the barrel would allow the bullet to have a much greater chance of “wobble” around the x and y axis (if z is forward) than the cylindrical section of a normal bullet allows. Best case, uncontrolled wobble kills accuracy. Worst case, bullet could bind in the barrel, causing it to blow up in your face or ruin the barrel.

Third, if the back of the bullet were rounded as well, the propulsion would be packed in the gaps between the bullet and the shell. I’m guessing it would be much more difficult to control the explosion in a tight wedge shaped crevice than a plain cylinder with a cap on one end. You could have delayed uneven burns at the rear of the bullet that could cause all manner of problems.

11

Another WAG. If the bullet is travelling at a supersonic velocity then low drag would only be a factor near the end of the flight. Perhaps the extra room for propellant provided by the flat end is a good trade off for more power at close range.

12

scr4 Wouldnt this drag, in addition to the spin from rifling, improve stability (prevent wobble/tumbling) ?

13

Why not just make the cartridge longer? Loads are determined by velocity, and cartridges are designed to allow a little wiggle room for those who load their own, so it’s not a real low tolerance anyway. And it would be a simple matter to design the original cartridge longer wouldn’t it?

You don’t need to make it football shaped do you? I’m thinking of a slug exactly the same shape as the curent ones, with exactly the same gap. Just with a little extra metal added to the trailing end to reduce drag. More sausage than football.

Your third point makes sense, except that some bullets already have a slight tapering that I would guess would create this problem anyway. It never seems to be a problem.

antechinus, if what you say is true, why do subsonic handgun rounds retain the same shape?

14

Yes, and I think that’s why the manufacturers DON’T make them that way. Do you want the bullet to have the slightest inclination to tumble on its way to the target, creating all kinds of wild turbulence and maybe going off target?

The standard way makes the big end stay in the back where God intended it. This is what the comment about the arrows and the feathers meant.

15

(Offtopic) The Puckle Gun was able to fire square bullets. Just, ya know, in case you wanted to shoot square bullets.

16

Cardinal, isn’t it the rifling that is responsible for decreasing tumbling, rather than the shape? I would think that any long shape would have about the same tendency to tumble. Are you saying that the flat end works like the feathers on a a dart?

17

Which makes for the path of least resistance for the bullet? Having the sloped end in the front, or in the back? I’m guessing that this is one of the reasons that the back end is flat. It WANTS to stay back there. Running flat-back is the natural way for something to travel through air or water.

The rifling imparts a heck of a gyroscopic effect, and makes the bullet fight wind currents, because there is great resistance to changing the direction the bullet is pointed. The wind might push it righr or left, etc, (and you read about snipers trying to adjust for this), but at least the bullet probably won’t just get pushed sideways in the air, travel momentarily with the flat end to the side, and end up tumbling all over, losing distance and probably even more accuracy.

So, yes the rifling helps control the tumbling, but I was thinking that having the back be flat was another way to help that happen.

Are there any really knowledgable gun people here? I may email this thread to one.

18

Whoo boy. A lil’ too much supposition goin’ on here. :smiley:

Okay, first, as already noted, bullets come in all manner of shapes and sizes. A .45 ACP FMJ (full metal jacket, IE, a military “ball” round) is a sort of tall dome shape: Flat base, cylindrical bottom half, rounded nose. It’s perhaps only half again longer, if that, than it’s diameter.

On the other hand, you have something like a “spitzer” bullet from a .30-'06 or similar rifle. This is the classic long, thin, severely tapered and nearly sharp-pointed bullet. It’s over four times longer than it’s diameter, and most are “boat tailed”- there’s a truncated cone base.

Why the difference? Simple- the .45 isn’t meant to be a long-range round. Decent accuracy inside fifty yards is fine. The rifle round, however, is intended for maximum accuracy and retained energy at extended ranges. Therefore improved aerodynamics is required.

Second, and most important: While the “football” shape may indeed be one of the more aerodynamic, a bullet needs what’s known as “bearing surface”. This is the straight-sided portion of the projectile that actually comes into contact with, and is lightly swaged into the rifling grooves of, the barrel.

To a certain extent, the longer the bearing surface, the more stable the projectile will be. (Too long, of course, adds additional drag and wear.)

The .45 can get away with a short bearing surface due to it’s relatively slow velocity (700 to 900 fps) and limited range. The rifle bullet travels much faster (2,300 to 3,200 fps, give or take) and usually has a faster “twist”, or steeper rifling (usually expressed in inches per turn- a 1:16 will make one complete revolution of the rifling grooves in sixteen inches.)

Thus, the rifle bullet needs more bearing surface for proper stabilization, and to a lesser extent, to keep the bullet from being blasted straight out before it can “grab” the rifling and begin to revolve.

So there’s a limit to the shape of the bullet- plus, as noted, the gasses from the propellant need to have a flat surface to push against. Rounded bases tend to be leak-prone and inefficient.

Why not make the cartridge longer? Well, they do. Frequently.

At the turn of the last Century, we had the .38 S&W short. To get more power, it was lengthened slightly to the .38 Long. Later still, in the thirties, it was lengthened again to make the .38 Special. By 1958 it had been stretched yet again and given modern powders to make the .357 Magnum (which, despite the name, still uses the same .357" bullets as the .38s have since they stopped being heeled rounds.) More recently, it was stretched yet again to make the .357 Maximum.

The .44 Magnum is a stretched .44 Special, itself a decendant of the .44 Russian. The .45 ACP was stretched and made into the .45 Win Mag. Ad nauseum.

However, case volume limitations still exist, besides the aforementioned bearing surface question.

19

Thanks for that Doc.
Why can’t they make a sausage shaped bullet simply by adding a peice of tapered metal onto the base? Wouldn’t that allow exactly the same bearing surface as the bullet it was made from, and still allow better airflow?

20

Yuurp. That there’s a bullet coming mahhh way. Thought at first might be a grasshopper, but now, aahm pretty sure it’s a bullet. Better duck, huh?