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  #1  
Old 06-09-2005, 02:18 AM
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How does passing through the sound barrier affect a bullets' flight?


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What happens when a bullet goes through the sound barrier, for eg. when a long range shot is taken and the bullet's velocity drops to below the speed of sound? Is the bullet more unstable at this point?
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  #2  
Old 06-09-2005, 05:46 AM
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Oh man, that is a COMPLICATED question. What happens when a bullet crosses the sound barrier ? A small amount of buffeting, just like any other body passing through this velocity region. If the bullet is properly constructed (well balanced and concentric)....and is spinning at enough RPM to stabilize it...then, very little displacement of it's trajectory will occur. A bullet, (as it goes supersonic when fired), is, of course, still in the barrel, so there is no disruption of the flight path. As the bullet slows, passing again through the sound barrier (in the OTHER direction), some instabiity will occur...which is mostly counteracted by the spinning effect. At extreme range, when the bullet has lost most of it's velocity (and most of it's spin)....it will become unstable...and ultimately, start to tumble. However, in the case of the average centrefire rifle bullet, because of bullet drop, the round will strike the ground (in most cases, depending on the height above the ground from which it was fired), BEFORE it loses that much velocity and spin.

The trick to it is to design things so that the velocity (and spin) is adequate, all the way to the target. This entails a maximum "effective range" for each particular cartridge. Anything that can be done to prolong the velocity of a bullet also helps. This is why boat-tailed bullets are much better at long range...but no real advantage close-in. They, because of reduced base drag, lose velocity more slowly than flat-based bullets.
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  #3  
Old 06-09-2005, 05:53 AM
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I don't know about the physics of flight. But I know about the sound. When a bullet passes near you above the speed of sound you will hear a sharp crack. It's too late to duck then, the danger is past. When a bullet ricochets (gets damaged) and slows down below the speed of sound, it buzzes as it passes by. Sounds like a big bumble bee. If you hear it, it missed you.

These are just a couple of pearls I picked up from "class" during my Junior Year Abroad Program in Southeast Asian Studies, 1968-69. I was funded by the U.S. Marine Corps: no tuition, monthly stipend, clothing allowance, and all the ammo a boy could want. ;-)
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  #4  
Old 06-09-2005, 06:00 AM
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Ooooooooo....this one will make you think.

I'll make a stab at this. My opinion is it will have little if any effect on stability.....why.

Breaking the sound barrier means you are traveling faster than the speed of sound and creating pressure waves by compressing the air via directional travel. Compare a bullets flight to an aircraft....an aircraft is a large irregular shaped mass that does not benefit by roatational stability. It relys on thrust to create a pressure diffential to provide lift. Stability is achieved via its aerodynamic shape (fuselage, wings, tail, etc). Going through the sound barrier (entering into or decensding from) will result in instabilities in flight for a short period of time because of the effect of the compressed wave (pressure) over the irregular shape. I have no first hand experience in this but I would imagine if you fly aircraft capable of breaking the sound barrier you want to make the transition quickly into a more stable environment whether accelerating or decelerating.

A bullet on the other hand is symetrical in shape, it has small mass, and is given a stabilization vector (rotation) to minimize, if not eliminate altogether, the effects of speed through air whether it's traveling at supersonic velocities or not.
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Old 06-09-2005, 08:14 AM
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Indeed it does have an effect, and it can even start before the bullet drops below the speed of sound - "transonic," I believe, is the term used, as the bullet slows and approaches the sound barrier.

There is always some loss in accuracy, hence the goal of keeping bullets supersonic all the way to the target, in things like Palma (1,000 yards) shooting. So those shooters use long-barreled rifles and pretty hot loads to keep the .308 Win supersonic at that distance, for example.

However, it is not hopeless; look at the black powder sihouette crowd. They're definitely not staying supersonic to those 500 yard rams!

There is a definite concern regarding twist rate, but that has more to do with the arching trajectory than actual velocity. "Gyroscopic predecession" (believe I spelled that right) is a term that deals with whether the bullet comes down from the arching trajectory with the nose pointed where it is going, rather than at too high of an angle. For example the Sierra 155gr. "Palma" bullet specifically requires a 1-13" twist to properly stablize at 1,000 yards. For normal hunting range, a faster twist works fine (i.e. the usual 1-10" twist) but not so at these long ranges. The extra drag can cause the bullet to drop below supersonic speeds sooner than the ballistic charts would indicate.
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  #6  
Old 06-09-2005, 09:23 AM
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A bullet is most unstable at the speed of sound. If it can make it through the sound barrier without tumbling, it will maintain it's accurate flight. The 1000 yard black powder shooters are proof of this. So there's two solutions to the problem. Use a bullet and load that will stay supersonic all the way to the target, or use a bullet that will stay stable though the transonic zone.

Shooters using modern centrefire rifles use the first method and gain the benefit of much less wind drift. Black powder shooters have to fight the wind, and have to give up a percentage of the bullet's ballistic coefficient to gain the necessary stability. So more wind drift.

Go here:http://www.eskimo.com/~jbm/calculations/drag/drag.html
and run the default calculation. Note that stability drops below 1.0 in the transonic zone, which is bad. Now take off the boat tail, by changing the length of the boattail to zero and changing the base diameter to .308", and recalculate. The ballistic coefficient drops, but stability stays above 1.0. I know there's other factors involved here, but you get the idea.

The Palma Match is somewhat restricted by it's rules which requires the barely adequate .308 Winchester. So we see loads that are right on the edge or even a bit over. Other 1000 yard matches don't restrict the cartridge as much and better cartridges are popular. I'm not really up on what's current, but the 6.5-284 was popular for the Wimbledon and Leech Matches a few years ago and the .300 Weatherby ruled 1000 yard benchrest.

Over stabilization isn't the answer, as accuracy suffers and the tip-over problem is real at long range. IIRC, you don't want a stability of over 3.0.

Check out this site for more on a very complicated subject.
http://www.nennstiel-ruprecht.de/bullfly/

Bye
Jack
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  #7  
Old 06-09-2005, 01:52 PM
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Here's another site with lots of good info on bullet flight. I haven't read this for a few years, but you might find lots of good answers on what affects bullet flight most.

http://www.fulton-armory.com/fly/index.htm

Good Luck, gene


Looks like Jack's latter site above is an update on mine, so I'll have to bookmark his.

Last edited by EDip; 06-09-2005 at 01:56 PM.
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  #8  
Old 06-09-2005, 02:12 PM
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You brought up an interesting question. Little off topic but still on. If you ever notice a .22 if it is shot some distance away from you, you get that crackle of the bullet in flight that is it breaking the sound barrier. Now something from my past. When I was in the benchrest fun and games, we used to have live turkey shoots the bird was tethered in a pit and you had to shoot it in the head at 200yds not a real reliable target they would move at an inoportune time. Back on track, when working the pit for the turkey shoots we stood behind a berm along side the range and in front of the back berm. The only sound we would hear from the bullet would be a sharp snap as it past by us, then the report of the rifles muzzle blast. Reading about the 1000 yd matches I once got invited to Grayling, Michigans National Guard training area they have a 1000yd range there and a fella was shooting a .308 win while I took a turn in the target pit. Some of his rounds would stick in the target (the target backing was 1/2 thick fiber) just one of my past expieriences. The bullet would make a sound much like a wet towel being slapped against a brick wall in a gyms locker room wall, when it struck the target. Bottom line it probably does effect the bullet but the same way each time. Guess thats why we go for a narrow spread in fps for our handloads, so we break the sound barrier consistantly.
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  #9  
Old 06-12-2005, 03:25 PM
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Personally, I wouldn't be shooting at an animal with a 30-06 whose bullet was dropping below the speed of sound after 1000 yards.

Now a 30-30 is a different story where the bullet my break back through the sound barrier around 200 - 225 yards.
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  #10  
Old 06-12-2005, 04:15 PM
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Quote:
Originally Posted by 147 Grain
Personally, I wouldn't be shooting at an animal with a 30-06 whose bullet was dropping below the speed of sound after 1000 yards.

Now a 30-30 is a different story where the bullet my break back through the sound barrier around 200 - 225 yards.
According to Hornady a 30-30 can push a 150 grain bullet to 2300 fps with a B.C. of .186 which means that bullet would not slow to the speed of sound until 350 yards according to my ballistics program at 350 yards the velocity would be 1144 fps at sea level. If I am not mistaken 1142 fps is the speed of sound. It is my understanding if the twist rate is correct once the bullet transends below the speed of sound then it will restabilize,but will lose some stability as it passes through the sound barrier just as an airplane does
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