Bullet "instability" question

[Tim Singleton]

Where's the Berger bullet technician guy when you need him

 

[ballison]

Catshooter,

After a good night sleep to reflect on our debate, I realized that my biggest error may have been one of semantics. I used the word “turbulence” to describe what makes a bullet unstable when it goes transonic, when I maybe should have used “disruptive forces”, or something else. Whether it’s turbulence, change in CP, compressive/expansive waves, cavitation, laminar/turbulent flow, …. or any of the other shit we thought was boring in school, there is stuff that happens in the transonic zone that is much harder to model, predict, and repeat, than if you stay in the supersonic or subsonic zones. And isn't that what we, and the OP, are all looking for? Predictability and repeatability? If nothing else, I was hoping we could all agree on this.

A bullet that stays supersonic the whole way to the target has more repeatable characteristics than one that enters the transonic zone.

 

[Road_Clam]

Road_Clam - do yourself a favor and purchase a copy of this book:

http://store.appliedballisticsllc.com/ProductDetails.asp?ProductCode=0001

Done !

 

[CatShooter]

That is not the nose wave moving back - that is impossible. The nose wave just fades wheen the bullet crosses 1085...[..]

"... and your twisting what I wrote (the Transonic region shockwaves shift), I said nothing of the nose wave moving back, that be your twist."

You have a short memory...

More to the "myth"....

"... the Transonic region shockwaves shift from the tip of the bullet backward to the tail as the bullet approaches and then crosses the sound barrier at Mach 1.

I rest my case!

 

[Syncrowave]

The "Trans-sonic zone turbulence" is a myth

I suspect Bryan Litz, who has forgotten more about all of this than I will ever learn, would disagree.

 

[Syncrowave]

Does the instability grow more pronounced as the bullet travels greater distances ?

My understanding is that at longer ranges, so long as the bullet remains above about Mach 1.2, the bullet actually becomes more stable and less liable to keyhole because while the "tipping force" that tends to make the bullet keyhole decreases with velocity (you can see this by angling your hand out your car window at different speeds), the rotational speed that gives the bullet gyroscopic stability remains almost the same in RPMs.

 

[Joe Salt]

I've seen bullets go sideways at 1000 yards but never at 100! So if this is happening at 100 yards you have real bad bullets or aeal bad barrel. At 1000 yards its mostly because the twist is to slow and the velocity at the start from the barrel is to slow. A bullet needs spin to stay stable.It doesn't have finns like a torpedo going threw water. Or how about a Gyroscope when it quites its spinning what happens. so if the bullet stays in the air long enough and has lost its spin its clear enough to me its going to tumble.

Joe salt

 

[CatShooter]

I've seen bullets go sideways at 1000 yards but never at 100! So if this is happening at 100 yards you have real bad bullets or aeal bad barrel.

Joe Salt

Bullets are a well known brand, 50gr, not too long for the 14" twist., and barrel is okie dokie.

Same rifle, same day...

 

[M-61]

I've seen bullets go sideways at 1000 yards but never at 100! So if this is happening at 100 yards you have real bad bullets or aeal bad barrel. At 1000 yards its mostly because the twist is to slow and the velocity at the start from the barrel is to slow. A bullet needs spin to stay stable.It doesn't have finns like a torpedo going threw water. Or how about a Gyroscope when it quites its spinning what happens. so if the bullet stays in the air long enough and has lost its spin its clear enough to me its going to tumble.

Joe salt

Joe, I have a .222...older BR rifle...Hart barrel (1-14), sleeved Rem action, SHBF. Had it made in the 70's, and it is a nice repetitively accurate rifle. 52gr Sierra HPBT 20.8gr 4198 and you are there. One day some time ago I decided to try some 69gr Sierras at 100 yards. 5 shots and one will keyhole nearly every time. The group is horrible. That weight bullet, in that rifle, with that twist will give you your key holed bullet one out of 5 times at 100 yards. I say this only because this will give you the worst results you have ever seen from a rifle whose barrel is fine but that weight with that twist would win a prize if you HAD to produce a key hole at 100yds.

 

[Joe Salt]

Catshooter $hit happens! All I can say is if it keep happening you got a problem, but even my shotgun with slugs has never done that. Are you sure you didn't kill a bird or something on the way. I've seen that happen at 1000 yards someone hit a swallow, needless to say the bullet never made it to the target.

joe salt

 

[Joe Salt]

M-61 If you are not spinning the bullet fast enough its like shooting it out of a smooth bore!

Joe Salt

 

[Syncrowave]

I've seen bullets go sideways at 1000 yards but never at 100! So if this is happening at 100 yards you have real bad bullets or aeal bad barrel. At 1000 yards its mostly because the twist is to slow and the velocity at the start from the barrel is to slow. A bullet needs spin to stay stable.It doesn't have finns like a torpedo going threw water. Or how about a Gyroscope when it quites its spinning what happens. so if the bullet stays in the air long enough and has lost its spin its clear enough to me its going to tumble.

This is from Bryan Litz's "Applied Ballistics for Long Range Shooting":


"As a bullet flies downrange, its forward velocity declines much faster than its losing rotational velocity. This means that the aerodynamic overturning torque diminishes at a faster rate than the rigidity of the spinning mass, so the gyroscopic stability of the bullet actually increases over its flight...Consider a bullet launched with marginal stability (lower than recommended twist). This bullet will fly with some amount of pitching and yawing until the gyroscopic stability (Sg) improves enough (thru loss of velocity) to restore point-forward flight. This phenomenon is often described as the bullet going to sleep (if the twist is too slow, the bullet will simply tumble instead of going to sleep)."

 

[barefooter56]

Guys,
gstaylorg nailed it. Buy Bryans book(S) He's a nice guy, his credentials are impeccable as a shooter and an engineer and he puts it in way everyone can understand. And being one that could do the experiments but not the formulas in High School I really appreciate that. 8). His new book Ballistic Performance of Rifle Bullets is outstanding also. I am wading through all 4 in order. They are NOT a dry read either.
Take care,
Phil Hoham
Berger Bullet Tech