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When the bullet goes transonic/subsonic

 
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Old 08-23-2007, 09:37 AM
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When the bullet goes transonic/subsonic

Hello everyone

I have read much about supersonic range, and that when a bullet goes into the transonic-zone, and in the end going subsonic, it looses much of its stability and the BC drops to a much lower number. But what does excactly happend to the bullet, I mean, other than being more affected by wind? Will the accuracy decrease, even if there was no wind to affect the bullet? And is it possible to calculate and predict the path of the bullet during the transonic area, using some kind of formulas or a ballistic computer?

I have read somewhere that some 50 caliber bullets, like the 750 grains AMAX (with a BC of 1.050), are very stabilized compared to other bullets when going transonic. Is this true? I have also heard the same thing about the M33 Ball, but the M33 Ball does only have a BC of 0.670. But I think that Carlos Hathcock (the legendary USMC sniper) had a confirmed kill at 2500 yards, using a 50 cal M2 machinegun firing M33 Ball ammunition (or pherhaps it was M2 ball, but these two bullets have the same BC), and at 2500 yards, the M33/M2 ball would have been in the transonic zone for a long time? How did he manage this shot in that case?

Sako75
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Old 08-23-2007, 12:21 PM
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This is an incrediably complicated subject. I myself do not have all the answers in this regard or 1/2 the answers for that matter. I dont know anybody that does. What I do know is that most standard projectiles will loose enough of their stability at the transonic wall to cause them to wobble. The wobble gets increasingly worse and then they begin to "tumble". First they will "key hole" into your target. This means they hit nose first but at an angle. When they tumble they ussually hit the target sideways. If an airplane didnt have wings and other stabilizing factors, they would do the same thing.

Some bullets will react different than others. Depending on balance, the stability factor of it due to the twist rate used and other factors some may be able to withstand the forces of the transonic wall and even though they are affected by it will regain their stability after reaching subsonic velocities. A short for heavy weight ratio bullet spun very fast will typically make it through the wall and keep going. Some bullets also increase in BC after going subsonic. Why does this happen. I honestly dont have the answers to those questions. I just know some will do different things with different twist rates. Overspinning a bullet though leads to other issues also so I personally wouldnt recomend getting a new barrel with an ultra tight twist either.

As far as Hathcock's 2500 yard 50 cal. kill, it is hard to know just what the real BC of the bullet was and we can assume it was close to published but do we know the drag model of it? Depending on the drag model it could have been super sonic past 2500 yards in the enviornment he was shooting in. Also depending on the twist that was used in that rifle, had it gone subsonic, it likely will have pushed through it and reached a subsonic stability and continued its flight.

Clear as mud right?
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Old 08-23-2007, 03:02 PM
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VERY complicated subject for sure Meichele, with more variables than can be reckoned with for me to theorize.

Below is a pic of my 24"x33" AR-500 steel plate, covered with bullet marks from 1,150 yards. The bullets that struck sideways are .308 168 grain Federal Gold Medal Match factory ammo, fired from a 26" PSS. They had still been hitting nose-on out to 1k, but fell apart not long after. Even sideways, at least some of them stayed relatively true to the aiming point. Everything else on the plate was still stable at that range and air density, and ranged from .243Win/115 DTACs to 6.5-284/Berger 140s, .300 WinMag Berger 210s and .308 175 FGMM factory ammo. IIRC, density altitude that day was around 2,500'.

At Quantico MCB, VA, where altitude density is more often around 500 to 1000', the 168s will be often be sideways as close as 800 yards.

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Old 08-23-2007, 06:01 PM
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One thing you have to keep in mind is that Hathcock shot a Viet chong solder at 2500 yards with a M2. It isn't proven that he got him in the first poke. ALso This wasn't the first time that Hathcock shot the m2 at range with his scope mounted on it. My point is that he could have easily sent several hundred rounds down range with the setup before actually making the hit at 2500 over the period of several months. He was shooting off of a fire base, not a sniper nest so he could have been (and most likely was) practicing for quite some time.

Don't get me wrong, I know he was one of the best snipers the US has ever had, but you have to look at things the way they were.
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Old 08-23-2007, 08:00 PM
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Yeah I'm gonna assume the whole sniper thing is pure BS..

If you look at any drag curve, you'll see that drag peaks in the transonic region. It's lower subsonic. It's lower supersonic.
Picking up & letting go of that shock wave is a huge, destabilizing change.
It's been identified in much out of Aberdeen that any bullet's performance in this region is unpredictable. Normally seen as a disclaimer.
It simply has to be tested to know, as with most anything aerodynamic.
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Old 08-23-2007, 08:06 PM
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Quote:
Originally Posted by Mikecr View Post
It simply has to be tested to know, as with most anything aerodynamic.
That sums it up VERY well.
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Old 08-24-2007, 12:35 PM
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britz +1. In the back of one of Elmer Kieth's books he mentions that there is a lot of talk in the book about what he had shot with his pistols; it doesn't talk about how much he missed. If you shoot at something enough times eventually you will hit it. I meen this in no way to disparage Hathcock at all.

Also, I don't know anything about it but I have heard the rimfire guys talk about keeping their bullets subsonic because of stability issues.
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