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  #1  
Old 11-28-2012, 04:42 PM
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Bullet Acceleration and Sound Barrier


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Hello,

Can someone comment on WHERE a supersonic bullet actually breaks the sound barrier? Does this happen while the bullet is still traveling through the barrel, as it leaves the barrel, or after it has completely left the barrel?

Thanks. Any explanation would help. My guess is that the bullet is still accelerating as it leaves the barrel, but once it does actually leave the barrel, the acceleration quickly drops to zero within a short distance of the barrel as compared to the overall horizontal distance the bullet will travel.
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Old 11-28-2012, 05:27 PM
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Welcome to the forumTMK044 and good question.
The bullet is accelerating up to the point it leaves the barrel and in the barrel it passes the speed in which to break the sound barrier but until it is in the open air you don't here it. At sea level it needs to be moving at 1127 fps to break the barrier, and at higher elevations say 2000 feet and up it will break the barrier at about 1116.4 feet per second. I think those are the correct speeds if I remember correctly, if I'm wrong it won't be by very much and someone will correct me. The gases from the powder burning actually leave the barrel 2-3 times faster than the bullet does. It's when the air passes over the bullet and off of it's tail end that creates the noise IIRC. The reason you hear the noise for so long is that it is breaking the sound barrier during it's entire flight unless you are shooting an extremely long way. There are a number of cartridges that stay supersonic out to 1000 yards and beyond, so as long as it is at that speed and above you will here that really cool sound it makes as it is in flight. Hope this helps !
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Old 11-28-2012, 05:32 PM
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The bullet stops accelerating after the the base of the bullet exits the muzzle. Depending on the the case capacity of the cartridge the bullet will reach supersonic speed within 2 to 3 inches of the chamber inside the barrel. For most cartridges it will take 4 to 6 inches. For many shotgun loads it will take the entire length of the barrel to reach supersonic speed. Small capacity cartridges like the .45ACP will never see its bullet reach supersonic speed in hand gun barrel less than 6 inches. However some handgun cartridges used in Thompson Contender length barrels (10 plus inches) with small capacity, can see their bullets reach supersonic speed by the muzzle.
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Last edited by mr glo; 11-28-2012 at 05:40 PM.
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Old 11-28-2012, 08:34 PM
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Actually, the bullet (I'm speaking of high power centerfire rifles here; I don't believe I know about handgun cartridges or rimfires) continues to accelerate until the base of the bullet is a few to several calibers beyond the muzzle. This has been verified many times with good equipment. Check Harold Vaughn's work or Robert McCoys for more details, as I'm not in a place to quote from the references just now.
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Old 11-28-2012, 09:26 PM
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Interesting topic that brings up more questions for me. I have heard this before, but this last elk hunt, I noticed a peculiar phenomenon more than in the past. When forward of another hunters gun when fired, and to the side (in this case, many hundred yards away), I heard a distinct 'crack', then a 'boom' shortly after. I assume the 'crack' is from the gas/bullet leaving the barrel, then the 'boom' is from the supersonic 'boom' pressure wave from the bullet out of the muzzle?

I haven't been shot at before, but imagine those that have been (in the military, or otherwise) have heard this too?
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Old 11-29-2012, 03:53 AM
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My perception, when I was several hundred yards down range from the shooter and the bullet coming my way I heard what I'd call a ZZZZip then bang. I labeled the ZZZip to the bullet moving thru the air a head of breaking the sound barrier.
If the bullet passes some distance from me all I hear is the sonic bang or the fire arm going off.
It's my understanding the bullet creats a sonic boom all along its trajectory until it goes subsonic. The reason for the lag between the ZZZip, pause then bang, is the ZZZip is newly created ??????
It's an eery sound.
OK now that I've totally baffled myself, any one have a guess what I'm speaking of?

Jim
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Old 11-29-2012, 10:15 AM
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Quote:
Originally Posted by arkypete View Post
My perception, when I was several hundred yards down range from the shooter and the bullet coming my way I heard what I'd call a ZZZZip then bang. I labeled the ZZZip to the bullet moving thru the air a head of breaking the sound barrier.
If the bullet passes some distance from me all I hear is the sonic bang or the fire arm going off.
It's my understanding the bullet creats a sonic boom all along its trajectory until it goes subsonic. The reason for the lag between the ZZZip, pause then bang, is the ZZZip is newly created ??????
It's an eery sound.
OK now that I've totally baffled myself, any one have a guess what I'm speaking of?

Jim
Yes we do, it's just like a plane at supersonic speeds, you can hear the plane coming but until it goes by you, past you, you don't hear the sonic boom.
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Old 11-29-2012, 10:16 AM
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The crack is the bullet passing along at supersonic speed. The boom is the hypersonic sonic detonation wave in front of the exothermic reactions of the muzzle ejecta.
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Last edited by mr glo; 11-29-2012 at 10:18 AM.
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Old 11-29-2012, 10:36 AM
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Originally Posted by MZ5 View Post
Actually, the bullet (I'm speaking of high power centerfire rifles here; I don't believe I know about handgun cartridges or rimfires) continues to accelerate until the base of the bullet is a few to several calibers beyond the muzzle. This has been verified many times with good equipment. Check Harold Vaughn's work or Robert McCoys for more details, as I'm not in a place to quote from the references just now.
Just to expound on this thought as it is counter intuitive. Is that the bullet is travailing along in an equilibrium of gas. Behind the bullet is the expanded gasses from the powder charge. In front of the bullet within the barrel is compressed air from the outside atmosphere. So, form 10 to 40 micro seconds the bullet is encapsulated in “air” accelerating with the bullet as both exits the muzzle. Then the encapsulation breaks up, slows down and the bullet begins to experience air resistance and decelerates.
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Last edited by mr glo; 11-29-2012 at 10:49 AM.
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Old 12-06-2012, 03:43 PM
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Supersonic at ground level under average conditions is around 768mph.
I reckon most projectiles running over 1126.4 Feet per Second will do so at the muzzle since 768mph=1126.4FPS. Not sure of the question actually. The sound barrier is only broken when under atmospheric conditions. This means once the projectile becomes airborne and under the laws of gravity and atmospheric pressure. While the projectile is still in the barrel does not count as being under atmospheric conditions. Gravity takes effect on projectile immediately after it leaves the barrel. I think the rest of the rules or laws of physics apply at the same point. Interesting question though.
Welcome to the forum and nice intro with a brain teaser!
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Old 12-06-2012, 08:57 PM
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When a bullet leaves the muzzle the gases behind it are accelerated to still higher velocities because they are lighter than the bullet and are thus accelerated further by the same pressure. This also pushes a spherical atmospheric shock wave out ahead of it. That continues until expansion makes the pressure drop too low to overcome air resistance and the expansion starts to slow. The muzzle blast also plays off the bullet base as the bullet first emerges, adding acceleration to it as well. Typically about 3% of velocity is gained in post-muzzle acceleration in high power rifles. That was also about the amount gained on a Doppler radar trace of a 40 S&W pistol round I saw.

Because the gas out accelerates the bullet, so do the muzzle blast and spherical shock wave. The bullet actually has a brief tail wind. No shock wave comes off the bullet itself while it's inside the expanding sphere because that requires exceeding the speed of sound in the gas relative to the movement of the gas, which is expanding with the bullet.

As that expansion gets larger and slows, the bullet catches up with its leading edge and breaks through it. At that point it is in regular atmospheric air, and relative to that, it breaks the sound barrier. In the second Schlieren photograph on this page, you see a .30-06 rifle firing. You can see the conical shock wave of the bullet that has formed as it has emerged from the expanding muzzle blast shock wave sphere whose expansion has slowed enough for the bullet to surpass it. It has caught up and escaped the front end of that expanding sphere.

When you are in the target pits at Camp Perry you are below the berm and shielded from direct sound, which attenuates it. As the bullets go overhead you are exposed to their shock wave directly for just a few feet of bullet travel before they pierce the target. The sound is about like firing a .22 pistol. It's a sharp crack and you want your hearing protection on. If it's just one shooter you can distinguish the boom of the muzzle blast arriving afterward, but with distant loudness. You only need to have this experience once to be driven to disillusion with TV silencers on high power rifles whose shots find their mark in total silence.
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  #12  
Old 12-12-2012, 05:27 PM
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well, It depends on how you phrase your question.... the bullet is always subsonic with reference to the gas behind it while in the barrel (the gas expands at a subsonic rate while in the barrel) though it could be supersonic (that is, there is a shock wave in front of it) the reference to the gas in front. But, if you are talking with respect to ambient conditions, a standard rifle round "breaks" the sound barrier inside the barrel. BTW... speed of sound is proportional to the square root of the temperature. So it is quite variable.

Most of that is to say... read unclenick's post, that is a good explanation of it.

Last edited by marineman; 12-12-2012 at 05:30 PM.
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Old 12-12-2012, 06:40 PM
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Thx Nick, and mr glo. This explains what I and others on the hunt were hearing. And, what you hear is dependent upon your position in whatever cone angle you are from the muzzle.
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Old 12-17-2012, 02:09 PM
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Very informative and very interesting. In all of the years I have been shooting rifles I have never actually gotten down to the physics of it all in this way. I have of course experienced the sharp snap as a bullet passes the two feet or so overhead in the butts and many times have heard the bullet 'slap' the target animal, usually before hearing the report from the rifle or the passage of the bullet. Thanks for explaining how it all happens.
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Old 01-05-2013, 08:52 PM
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There are a couple of reasons the listener's position is relevant here.

First off, if the listener and the shooter are very close, you're not going to hear any sonic boom at all. The shock wave will always be out in front if you, so there's nothing for you to hear, and the report of the gun (nothing to do with the shock wave) would completely drown out the sonic boom if it was there to hear.

This is why many folks have never heard the 'crack' you're referring to--they've never been in the proper position to. About the best way to hear it is to be well out in front of the gun. If positioned a good distance in front of the gun, the bullet and the shock wave will pass you well before you hear the report of the gun (which is travelling at 'sonic' speed, by definition).

Now, this all works best for rifle bullets which are significantly supersonic (2 or 3 times the speed of sound) and stay that way for most, or generally, all of their flight. The passage of bullets travelling this fast precedes the gun's report by a significant amount of time, and lets you clearly distinguish two separate sounds--the shock wave 'crack', and the gun going off.

Handgun bullets that are 'just' supersonic may not stay that way for very long, and the shock wave is generally much weaker. As unclenick mentioned, the shock wave is a spherical wave pushed along by the bullet, but this is more the case for bullets just making sonic velocity. For 'bullets' like a penetrator leaving the muzzle of an M1 Abrams, that wave is much more of a cone shape.

In reference to a question asked above, the shock wave that causes the 'sonic boom' travels with the object--be it a bullet or an aircraft. So, the shock wave itself travels past you at the speed the object is traveling (bullet speed or aircraft's ground speed). It is when this wave passes you that you hear it. You can think of it exactly like the bow wave of a boat--it's doing the same thing as far as how it's moving along with the object.

Likewise, this 'bow wave' has an angle associated with it--the greater the velocity of the object, the more swept back the angle. For a rifle bullet passing you, it will have to pass you for some distance before you hear the crack.

The speed of sound in any material (air, water, steel) is determined by the density of the material--sound travels faster under water than it does in air, and faster in dense air than in less dense air. So, speed record attempts on land that aim to 'break the sound barrier' will aim to run on a day with the speed of sound is the lowest--a hot day. If you're trying to go as fast as you can but don't want to go supersonic because of the amount of power it takes, than you run on a cold day. Likewise, a bullet that is just subsonic at low altitude might very well be supersonic at 6,600 ft where I live. And, taken to the extreme, you may recall on shuttle orbital recoveries, the news folks would comment that the "shuttle is travelling at Mach 18", or 18 times the speed of sound. Well, yes, it's going fast, but it's not travelling at 18 times the speed of sound that we know--it's travelling at 18 times the speed of where it currently is flying. At 150,000 ft above sea level, the speed of sound is probably 400 fps or something. So, 'speed of sound' is always relative to the medium the sound is travelling in.

Here's a handy graphic (no time or expense spared for my pals at Shooter's Forum). Shows the 'weak' bow wave of a slightly supersonic object vs the strong shock of a supersonic object, and how the event is experienced by someone out in front of the shooter. The further out in front of the rifle the listener positions himself, the wider the separation of the sonic crack and the report of the gun discharge. The gun discharge can only travel at sonic speed, (sound can only travel at the speed of sound), but the shock wave is travelling at the speed of the bullet.



This why you may have heard folks say "You won't hear the one that hits you." No, you won't.
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Old 01-06-2013, 07:05 AM
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Excellent technical description, nick - and equally good from Bongo.

I am fortunate to have been missed by perhaps half a million rounds of rifle bullets, all fired in anger. Those that missed by only a tiny bit made a sharp crack as they went by. There was always enough other noise going on that I never heard the hiss of the air disturbance caused by the bullet, and only rarely heard the muzzle blast due to range. But at times, the very quick sequence was: Snap! zzzzzzz boom.

No, you canNOT hear a supersonic aircraft approaching you. Because if you can, it isn't traveling faster than sound. You can't actually hear it until the bow shock wave passes you, and that shock wave trails the aircraft a bit like a boat wake. So - you can't hear a supersonic aircraft (or anything else) until it has passed you.
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Old 01-06-2013, 03:33 PM
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Originally Posted by unclenick View Post
When a bullet leaves the muzzle the gases behind it are accelerated to still higher velocities because they are lighter than the bullet and are thus accelerated further by the same pressure.
If you place your chronograph too close to the muzzle, it will give you a reading of the blast speed, and not the bullet speed. For example, it might show 2,500 fps for a 9 mm.
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Old 01-06-2013, 06:31 PM
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Thought folks might like to see an actual shock wave on an actual bullet:



Google 'schlieren' images for lots more, I'm sure. On this photo you can see a secondary shock behind the first one, where the airflow runs into a little upset at the crimp groove.

The reason these images can be seen is similar to the reason you see 'heat waves' coming up off an asphalt roadway or a card hood...changes in the density of the air create changes in how it bends the light passing through it, making a lens, in effect. This diffraction may also explain why a chronograph detects and measures the blast wave behind the bullet--that blast can present a striking and highly contrasting 'shadow', just as detectable as the bullet itself.

What's also pretty cool is that one can estimate the Mach number of that bullet by measuring the angle of the shock wave: in this case I get a Mach = 1.94, which at standard conditions would be about 2,183 fps.
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Old 01-07-2013, 06:17 AM
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As a complete aside, one of the coolest things I've ever seen was the shadow of my own shock wave on the wing of another jet while flying in close formation with him, supersonic. One of nature's own "schlieren images."
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Old 01-07-2013, 07:49 AM
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Now that sounds super-cool, Rocky!
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