Whats The Advantage of a Boat Tail Projectile?

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Now to understand how different shapes create different shock waves that produce wave drag on the supersonic object, we need to know the Whitecomb Area Rule. If you look at the boat-tail bullet images, you will notice the tail waves are faint -- so much so that many commentators ignored them entirely and must have believed they didn't exist. But looking at objects that more seriously transgress the area rule, like the T38C and that bullet two photographs above this post, we can see substantially more wave drag. The boat-tail has fewer and smaller shock waves, but a shape closer to the Sears–Haack body would be even more ideal.
 
Boattail bullets take a little more distance to stabilize in flight. Depending in the weight, diameter, and velocity, your flat base bullet should be more accurate at100 yards or less.
 
Flat-based bullets tend to seal gases better, so barrel life is greater. Other than that, I don't worry as much about trajectory as much as accuracy and how my rifle shoots a particular bullet.
 
Steelangel said:
There is nothing but reverberation behind the tip of the bullet. It's actually in a vacuum of space behind the initial shock wave. Where there's no air, there's no airflow.
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1. How does the shadowgraph reveal the initial shockwave? In other words, what is the mechanism by which a line is revealed on the shadowgraph where the initial shockwave is?

2. What causes the visible lines behind the initial shockwave and how does a shadowgraph reveal those lines?

3. What is the swirling behind the bullet--what causes it and what causes the swirling to show up in the shadowgraph?
 
taliv said:
one would think that given the same weight, the shank contacting the barrel would be significantly shorter, meaning less friction.
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It's kind of surprising, but friction does not depend on area. It's just force x coefficient of friction. Doesn't seem reasonable, does it?
 
JohnKSa said:
1. How does the shadowgraph reveal the initial shockwave? In other words, what is the mechanism by which a line is revealed on the shadowgraph where the initial shockwave is?

2. What causes the visible lines behind the initial shockwave and how does a shadowgraph reveal those lines?

3. What is the swirling behind the bullet--what causes it and what causes the swirling to show up in the shadowgraph?
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Because we cannot see the shockwave in a transparent medium like air, it is not observed directly. Because that disturbance does refract light, it creates shadows. To capture a still photograph of a short event at high speed, a very brief light source would be used. Early experiments used spark. The light refracts through the shockwave or other disturbance, and casts a shadow on a reflective background. It is the contrast on the background from the shadows that makes the differences in film exposure.

This was done as early as the 19th century. This one was slightly more complicated, a Schlieren photograph by Ernst Mach, published in 1887:

machbullet.jpg

The Schhlieren method was invented by August Toepler in 1864 to study supersonic motion. It shows the density gradients in the fluid distortions of a collimated light beam, using the shadowgraph principle. Notice that as early as these 19th century images, there was ample evidence that there is more than one shockwave generating wave drag on a supersonic object, and the effect of different shapes could be studied and verified.

The cause of the visible lines are the shadows of areas of compressed fluid refracting light. We refer to the boundaries of compressed fluid as "shock waves" or "pressure cones."

The swirling is similarly fluid distortions shown by the shadow of the refracted light.

The principle of the shadowgraph can easily be observed in the shadow of a fire during daylight. You might be able to use a common lighter flame held near a sun-lit wall to observe the fluid disturbance refracting light and casting shadows.
 
labnoti said:
The swirling is similarly fluid distortions shown by the shadow of the refracted light.
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I trust most of those reading this don’t know that air is a fluid. The vast majority interchange liquid with fluid

In engineering classes we had sessions in Fluidics, basically laminar flow, typically air
 
The mistake people are making in thinking there is no air along the sides of the bullet in those photos is they think that’s a wave of air being deflected away from the bullet tip. It’s not. It’s a pressure wave traveling through the air. Just like the V shaped wake coming off the front of a boat.

In fact those pictures are proof that the air is traveling along and around the bullet. The proof is you can see the pressure wave off the back of the bullet and the turbulence behind it. You cannot have a pressure wave or turbulence in a vacuum, therefore their existence is proof that there is airflow there.
 
redneck2 said:
In engineering classes we had sessions in Fluidics...
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Now you're just making up words.

Phlegmectomy. See, I can do it too. :D

(none of the following comments are directed at redneck2)

Two bullets, exact same construction, mass, diameter, ogive, etc... The only difference is one is flat based, the other has a boat tail. Shot out at the same velocity, the boat tail bullet decelerates at a slower rate than the flat based one. The only difference IS the aerodynamics of said bullets. (and possibly bearing surface, but that doesn't have a direct effect after the bullet has left the barrel.)

Cavitation doesn't happen in air. It also doesn't result in a vacuum in liquid. It results in an extremely low pressure area that is filled with the vapor of the liquid. As soon as the pressure returns, the vapor condenses and returns to a liquid state.

Matt
 
No idea why, but my short barreled 303 No4Mk1 (despite being a 2 groove) and 7.5 K31 both group better with SPBTs than flat base bullets. No scientific tests. Just lots and lots of different handload combinations. My best boattail loads edge out my best flat base loads. All of my hunting and shooting is done at 100 yards or less. So, that's what I hunt with. For me, it is what it is ;)

Be well all
 
Because we cannot see the shockwave in a transparent medium like air, it is not observed directly. Because that disturbance does refract light, it creates shadows.
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I know the answers, that's why I provided a link to an explanation of shadowgraphs in the questions. My idea was for Steelangel to answer the questions. :D

That said, your answer is very good. The lines in a shadowgraph are, exactly as you say, shadows of the density variations of the MEDIUM in which the density variations occur. In this case the medium is air. If it Steelangel's assertion about everything behind the initial shockwave being a vacuum with "no air" were true, there would be no medium, no air, present behind that initial shockwave and therefore nothing to create lines in the shadowgraph.

The lines that show up after the initial shockwave and the swirling lines that trail the bullet are proof that there is air, not a vacuum, behind the initial shockwave.
In fact those pictures are proof that the air is traveling along and around the bullet. The proof is you can see the pressure wave off the back of the bullet and the turbulence behind it. You cannot have a pressure wave or turbulence in a vacuum, therefore their existence is proof that there is airflow there.
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Correct.
 
FWIW, the U.S. Military went back and forth on its .30-06 issue ammo. After replacing the round nose with a spitzer flat based in 1906 (changing from the .30-03 to the .30-06 in the process), they went to a boattail cartridge in the 1920's and found that using it would cause rounds to overshoot old shorter rifle ranges designed for the m1906 cartridge. The Garand, WWII, and other exigencies led to reverting to something like the old WWI cartridge and calling it M2 ball.

If I recall, part of the issue is that machine guns were vastly more effective in range with a boattail round than not which was demonstrated by the French Lebel Balle D fired from the French Hotchkiss Machine gun. WWI era U.S. machine gunners were trained on the Hotchkiss and we used them in WWI due to the relative lack of the newer Brownings. Logistics always prefers one do it all ammo versus specialty niche ammo for a particular application.

https://www.thefreelibrary.com/FEED...+French+Machine+Gun+Influenced...-a0544829403 (this is from a June 2018 Firearms News article by David Fortier)

https://m1-garand-rifle.com/30-06/

Both articles, along with Hatcher's Notebook, have a lot on the specifics of why the military switches bullets in general.

In specific applications, a rifle may or may not fire boat tailed or flat based bullets accurately compared with the other type. Worn barrels are usually thought to prefer flat based but not always depending on the pattern of wear of the bore. Thus, YMMV.
 
Discussions like this where the uneducated claim that knowledge has passed to them through simple association with those who really do know

Steelangel said:
And I've worked with many company 'Enginerds' ...
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are fun, but generally only for their entertainment value. There has been enough unscientific conjecture and imprecise use of technical terms in this thread to keep me laughing for quite a while.

What's different here is that mixed in with all this dross, labnoti has taken the time to provide some facts (true, accurate, proven, scientific facts) regarding supersonic flow in compressible fluids. Thank you!

Two of his points deserve more emphasis:

First: many have missed the fundamental principle that pressure on the downstream side of a shock wave is actually higher than the pressure on the upstream side. Not a lower pressure (or even a vacuum) as Steelangel has proposed.
labnoti said:
Then scramjet engines would be impossible. And ramjets would also stall in the vacuum at supersonic speed.
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Scramjet and ramjet engines both depend on this principle of increasing pressure across a shock wave to eliminate or reduce the need for, and the power consumption of, the compressors used in conventional jet engines.

It is this pressure increase that creates the density differences that are visualized in the Schlieren or shadowgraph images.


Second:
labnoti said:
There might be some merit to the idea that amateur ballisticians have a misconception about the boat-tail being there to lower viscous pressure drag rather than understanding its effect on wave drag,
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Many of us have gained intuitive understanding regarding fluid behavior over the course of our lives, and some of this intuitive understanding may actually be correct! Drag reduction resulting from attaching panels to semi trailers and observations of boat wakes are two that have been mentioned in this thread (all examples of viscous effects). However, unless we've spent a lot of time testing SR-71s or artillery projectiles, then probably very little of our common intuitive knowledge applies to the behavior of bodies traveling at supersonic or transonic speeds (where wave effects dominate). Shooters' dope is as close as many of us come to intuitive knowledge, but even dope provides little understanding of the underlying principles of fluid behavior.

Supersonic flow in a compressible fluid is a VERY different animal than the subsonic flows that most of us have experienced (or studied). Only those who are ignorant of the differences try to extrapolate from one realm to the other.
 
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It's one thing to argue about the exact dynamics. But it was clearly demonstrated over 100 years ago that adding a boat tail to a projectile decreased drag. That's a fact, and anyone who tries to argue with it, contradict it, question it etc. is simply a loon who is wasting everyone's time.

From the perspective of the shooter, the reduction in drag is really all that matters.
 
Llama Bob said:
From the perspective of the shooter, the reduction in drag is really all that matters.
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This would be true at longer ranges. Inside maybe 300 yards, I believe there is usually minimal effect. From what I’ve seen, most shorter range target quality rifle bullets are flat base.

I’m loading up hunting bullets for my 7 Mag. They are BT’s at this point simply because I want as flat shooting as possible. Even if they’re just a gimmick. After all, what do those nerds at Sierra, Berger, or Nosler know?

:)
 
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redneck2 said:
This would be true at longer ranges. Inside maybe 300 yards, I believe there is usually minimal effect.
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Sure - drag in general has minimal impact at those ranges at least for target use. For hunting it drops your impact velocity, which may or may not matter depending on muzzle velocity and bullet type.
 
Think about NASCAR. It’s often said that aerodynamics at the rear of the car are as important if not more important than at the front. Disregardingthe need for downforce to help steer and propel the car your left with aero effect on the front and rear. The front has to cut the air, the rear is designed to minimize turbulence in the relative void along the back bumper, and the whole thing is meant to keep the car going in the exact right direction.

Now if we can just figure out a way to make bullets line up like a lead pack at Tallageda...
 
labnoti said:
More multiple wave drag conditions:

I already commented earlier that everything inside, behind the 'primary cone' is called resonance. What you're trying to show us as 'wave drag' is actually Resonate (Sound) Waves. Because the object is exceeding the speed of Sound. Don't remember studying anywhere that sound causes any significant drag.
Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass In principle, we cannot directly see a difference in temperature, a different gas, or a shock wave in the transparent air. However, all these disturbances refract light rays, so they can cast shadows.
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