Bullet Stability, or Lack Thereof

[BeeTee]

I don't believe the Nosler BTs are being "over-stabilized" in a Remington 700 223. A 223 using a 50gr BT bullet launched at 3450-fps in a 1:12 twist barrel will have approx the same spin rpm as a 220 Swift at 4000-fps using a 1:14 twist. I suspect even an AR with a 1:7 to 1:9 twist would still shoot the 50gr just fine.

In my experience, a bullet tumbles because of being under-stabilized. I once owned a mil-surplus 7mm Mauser with a bad bore that produced keyholes with every shot. The bad rifling wasn't spinning the bullet enough to stabilize it.

About a year ago I saw a heavy barreled Remington 700 in 204 Ruger sitting on the used rack at a local gun shop. The rack was on the floor with the rifle muzzles pointed up. I could see the bore at the muzzle for this rifle, which looked like it had been plated with copper. It was like looking down the inside of a copper tube. There was no visible rifling. I suspect bullets fired from this rifle would also tumble. BT

 

[Ridge_Runner]

I don't believe the Nosler BTs are being "over-stabilized" in a Remington 700 223. A 223 using a 50gr BT bullet launched at 3450-fps in a 1:12 twist barrel will have approx the same spin rpm as a 220 Swift at 4000-fps using a 1:14 twist. I suspect even an AR with a 1:7 to 1:9 twist would still shoot the 50gr just fine. BT

I know an 8 twist will, I have 223's twisted 8,9,10, and 12 and have no problems at all until I get over 64 gr in the 12, have shot 50-75's in the 8 and 9 twists with great results.
RR

 

[onesonek]

The decay of rotational velocity is virtually zero.
It has something to do with the law of conservation of angular momentum....

If you are fairly over stabilized or more to begin with, you won't notice the rpm erosion/degradation until likely a 1000 yds. give or take, depending. Or shooting a bullet that is real close to a GSF of 1 or slightly over, at shorter ranges. The Match/LR barrels all the Pro Comp guys I know, as well as myself and a few other guys play at LR for fun, set up around a twist for a few choice bullets of similar profile and MV. Not broad spectrum of weight or length and various amounts of MV. The Law of Angular Momentum Conservation doesn't apply as you propose it. It is rather interesting you brought up the Law however, as it is, allows the profressional ballisticians to compute the the erosion from a smooth projectile.
It is because the bullet is no longer smooth when it leaves the barrel. The land/groove striatations create rotational drag. Rotation erosion rate, depends groove depth, twist rate, velocity, and possibily a few more I can't remember off the top, but it's there. ( I had been calling degradation even though they mean the same, where erosion is the common name finally came to mind, bf likely)

If I'm wrong, it's because the best in the business have taught me wrong, since we went from a slide rule to Puter Programs

As a side note,
I hear tell that the Military's .338 Lupua is barreled with Polygonal rifling? I'm not sure as I can't find info on it's build right off. But if so, would it be a coincidence that it deforms the bullet less, creating smoother air flow/less rotational drag on the bullet?

 

[RiverRider]

It is because the bullet is no longer smooth when it leaves the barrel. The land/groove striatations create rotational drag. Rotation erosion rate, depends groove depth, twist rate, velocity, and possibily a few more I can't remember off the top, but it's there.

That sounds plausible in theory. I'd like to see what is actually known as to how much spin is lost at various ranges. Intuitively (at least for ME :!: ), it seems as though it could only be miniscule unless the bullet is in flight for several seconds. Is there any real data on this?

The more I think about it, the more interesting it gets. If a bullet's jacket allows the bullet's diameter to increase as it exits the muzzle due to centrifugal force, then the bullet's rotational velocity would indeed decrease in compliance with the law of conservation of angular momentum. But then if its diameter increases, then its length should decrease and then less rotational velocity would be required to maintain stability...so we might have a system here that keeps itself in equilibrium at all times, but then again, maybe not!

Sometimes I just go way too far down a path before I turn around. Sorry to be the cause of aching heads!

 

[onesonek]

It is small indeed but not therory, it's physics in action. At a 1000+ yds., every .01(s) moa makes a big difference between a hit or miss pending size of target. At extreme range, one also has to account for the Earth's rotation, which of course, is dependent on the shooters location in relationship to the equator.
I have no way of proving it, but according to my program's developer, whom told me in a phone converation, that it was developed for Aberdeen Proving Ground. But he could be just blowin smoke, but then again I really have no reason to doubt him. And in several other converstions with him actually, he seems highly knowledgeable and straight forward kinda guy. Also he said in side by side test, shooting cold shots at a 1000 meters from his and other programs, straight form the program's numbers, his was closer to accurate than any other tested. That said, all programs are definately not equal in their generation's of plugged in parameters. That's not saying there aren't better out there however. I just don't who's they are.
I'm trying to think if my program shows the actual erosion numbers, or its just in the program's final generation of parameters plugged in. Can't look right now as my son is using the lap top I have the program in. If not I'll look around, I know I have seen erosion comparison's some where, just can't remember where.
And sometimes I forget more than I have learned, making for mistakes or mistatements.
No need to be sorry, any questions or thoughts can be educational.
Again, all this is just what I been taught over the years. But I know also nothing is really is totally absolute, as nothing is subject to change with future learnings, regardless of subject matter

 

[Oldtrader3]

I worry more about the angular dispersion of my groups due to normal stress and strain harmonics of inertia being applied to my scope mounts during firing and bullet travel down the barrel. With two 6-48 screws in each of two mounts holding a 16 ounce scope on a semi circular surface while a couple hundred pounds of dynamic moment is applied to my scope mounts during normal firing, I begin to think that calculating coriolus effect is a waste of time. Without having my bases and rings welded to the action, this effect is greater than any downrange gravitational or even prestabilization pitch or yaw effects on the path of my bullet.

Just something to consider.

 

[onesonek]

No doubt you have brought valid points Charlie. And truthfully I don't get nervous about coriolis much, I just know its there, and something to consider at extreme range with cold shots. I don't think much on it out to a 1000. I always paid more attention to the wind changes at various ranges to the target.

 

[Oldtrader3]

That is right Dave, I worry about wind current vectors, wind speed and mirage, especially hunting in the western mountains and shooting cross canyon mostly.