At the beginning of my scientific studies relating to long range shooting and extreme long range shooting, I was learning about barrel twist rates. Fast forward a few years later, I am still studying about twist rates, but with an emphasis on how they have an affect on gyroscopic stability and bullet tractability. When choosing a bullet, I look at my barrel's twist rate and try to find a bullet long enough to be just stabilized. (On the low side of GSF 1.4 - 2.0) This is to not over stabilize the bullet and to maintain bullet tractability to make it through the transsonic zone. Let's go over a few terms to familiarize ourselves with these concepts.
There are a few terms referring to a bullet's stability, and it's important to know the difference. These are gyroscopic stability, aerodynamic stability, and dynamic stability.
Gyroscopic stability - The resistance of a rotating body to a change in its plane of rotation. The faster a body spins (the greater its angular velocity), the greater the stability of the body in its particular position or orientation. Effectively, it isa measure of how well a bullet is stabilized by spin.
Aerodynamic stability - This best describes a bullet that is in flight without yaw, is gyroscopically stable, and is at maximum ballistic coefficient.
Dynamic stability - The ability of a bullet to return to a steady state of operation after a disturbance.
It's important to know the difference between these stability factors. I have seen many people talk about a bullet becoming more stable further into the flight. This is partly true. Later in the bullet's flight, the yaw of a bullet has a chance to dampen out and become dynamically stable. However, if a bullet is marginally stabilized, this increases the chance thatthe center of gravity will surpass the center of pressure, causing the bullet to tumble. This isreferred to as "negative dynamic stability". So, now we have a general idea of how twist rate affects our bullet, and thatwe need somewhere between 1.4 and 2.0 when it comes to our gyroscopic stability factor. This leaves me with the question: How do I successfully shoot through the transsonic zone?
We know the transsonic zone can destabilize a bullet and is likely our maximum effective range. The transsonic zone is Mach 1.2, or about 1350 fps, depending on the environment. But, if we can successfully shoot through the transsonic zone, then we are only limited to the rifle's capability to shoot the smallest cone of fire. In my research, I have found two schools of thought. The first isto have the bullet only stable enough to maintain its stability and not tumble, giving the bullet a gyroscopic stability factor of 1.3 - 1.4. This allows the bullet to maintain tractability (bullet angle following actual flight path), and go nose first through the transsonic zone while remaining stable.
The second is what some refer to as "over spin", which is when there is significantly more spin on a bullet than what is needed to adequately stabilize the bullet. "Over spin" can create a gyroscopic stability factor closer to or over 2.0. This would be a bullet so dynamically stable that the disturbances of the transsonic zone have little to no effect on the bullet. However, with a shot at extreme long range, the bullet will pass its maximum ordinate and maintain the same angle as when it was fired. This means that,as it passes its maximum ordinate, it is falling with the nose at an upward angle, and is not aerodynamically stable. The result is thatthe bullet is entering the transsonic zone at such an angle that it would begin to tumble if it was not for its over-stabilization. But, because of the bullet's angle, its ballistic coefficient suffers tremendously.
Bryan litz of Berger bullets and Applied Ballistics point out that this is at least in part myth.
Per Berger's website, "the myth states that: spinning a bullet too fast can cause excessive stability and prevent the bullet from tracking with the trajectory, so it falls ‘nose-high’ on the downrange end, and causes more drag. This myth is easily dispelled if you recall the definition of stability, which is that the projectile aligns with the oncoming air flow. It does this even as the trajectory bends, and even for high SG’s obtained with fast twist. In fact, doppler radar data consistently shows that faster twist and higher SG’s actually reduce drag, especially at transonic speed for bullets fired at long range. The myth warning of overstability stems from high angle artillery fire, where rounds could sometimes fail to track, and fall base first to the ground. This is not a problem in the realm of small arms rifle bullets shooting at small angles (under 30 degrees)".
Lets look at some actual data that I have collected over a few years pertaining to this.
Let's first look at my .308.
Caliber - .308
Twist - 11.25 5R
Bullet - 175 SMK
Bullet length - 1.243
B.C. .474 G1
Bullet speed - 2650
SG - 1.88
I have "successfully" shot my .308 out to 1300 yards. Using my ballistic calculator, the bullet shows to go transsonic (1300 fps) at 915 yards. I can easily have great success with hits at 800 yards. At 900 yards, I do notice a slight change in hit probability. But at 1000 or more, there's a significant change and hit probability. I believe this to be the bullet's inability to successfully stabilize during the tranzonic zone.
Let's next look at my 6.5 Creedmoor
Caliber - 6.5 Creedmoor
Twist - 8
Bullet - Hornady 147 ELD-M
Bullet length - 1.43
B.C. - .315 G7
Bullet speed - 2589
SG - 1.54
The ballistic calculator shows my bullet going transsonic at 1135 yards. But I have easily had multiple hits, first round hits, and first round center hits at 1400 yards. In my 6.5, I'm using a longer, thinner bullet at slower speeds and a lower SG. All of these elements lean to a less stable bullet than what I use for my .308, and yet the results show that my 6.5 Creedmoor does significantly better through the transsonic zone. And at 1400 yards is going subsonic at 1080 fps and well into subsonic range (1132 fps).
And finally, lets look at my .338 Lapua Magnum
Caliber - .338 Lapua Magnum
Twist - 9.375
Bullet - Hornady 285 ELD-M
Bullet length - 1.743
B.C. - .417 G7
Bullet speed - 2615
SG - 1.96
All of my SG numbers were figured for 660 feet of elevation at 40°f. When the temperature increases so does the SG. So for my .338 to be 1.96 at 40°f, it goes up to 2.15 at 90°f. Thats way more spin than I want. But, what I saw shooting to a mile with this .338 lines up with what I suspect to be true concerning "over spin". My first shot was .5 mils low (off target). When I made the correction, the second shot was high (over the top of the target) and right. I split the difference between the two shots and sent a third round, hitting the plate on the bottom left edge. The point being, it was all over the place. My data shows my bullet going transsonic at 1427 yards. At a mile, that's just over 300 yards past the beginning of the transsonic zone. However, it's still in transsonic flight at 1760 yards. This could possibly show that the bullet is tumbling during transsonic flight whichcould explain the dispersion between shots. Or, maybe I'm just a bad shooter. More testing and time will tell, but as of now, I'm seeing that the bullet is over spinning and possibly becoming destabilized.
Per Berger's website: "If this ratio is greater than 1.000, it means the bullet has more stabilizing influence than de-stabilizing influence, so it’s said to be stable. In practice, a bullet needs an SG of 1.5 or greater to be well stabilized, and fly with the maximum effective BC".
If this is true, that explains at least part of the reason why my 6.5 creedmoor appears to make it through the transsonic zone. The SG is 1.54 vs. 1.88 with my .308 and 1.96 with my .338. But that's only one part of the equation. There are so many factors that involve a bullet's successful transsonic flight, such as all of our environmental conditions, but also things such as the rifling type and the number of lands and groves.
I set out to learn how to maximize my distance by shooting through the transsonic zone and have come to the conclusion that, as far as my research goes, there is no absolute or formula that will guarantee a smooth transition through the transsonic zone. It does appear however,that we can significantly increase the chances by having the SG at or near 1.5 vs. significantly higher or lower.
Unfortunately, the SG is only one part of the equation, but possibly our best option, to control the spin of the bullet to make the transsonic zone transition. Our second best option is the bullet itself. Bryan Litz of Applied Ballistics and Berger bullets have changed certain bullets' boat tail angle from 9° to 7°. A shallower boat tail angle shows more promise during transsonic flight. It also seems to be more favorable for a shorter and fatter bullet as well. Unfortunately, a short, fat bullet will not have as high as a B.C as a long, skinny bullet, but we can adjust for this increase in drop so in the end, it may not be a big deal.
With all this being said, there does seem to be one manufacturer that has the transsonic flight formula figured out, CHEYTAC!!!
Coming up in part ll, we'll look into what may be the best long range/sniper rifle.
The .408 Cheytac.
The Overwatch
The Overwatch
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