Re-Thinking Transonic Zone effects

[Lemonaid]

This Vid. by Mark and Sam after work changed my understanding of what happens in the transonic zone.

https://www.youtube.com/watch?v=lXODyVnE_Dw

I don't have any experience shooting the Grendel going subsonic so what say you?
Is there really not much effect on accuracy from properly stabilized bullets going from super to sub sonic?

[Arkhangel5]

Good vid, thx for posting.

I think the point he was trying to make was accuracy issues at distance are more likely environmentally caused than by the transonic transition.

SY

[A5BLASTER]

LRRPF52 has stated many times that a tight twist will help keep the bullet more stable I.E well behaved as it transitions from supersonic to transonic.

After shooting 1000 yards with both at and bolt action grendels with 8 twist barrels and then getting to try it with my 16 inch ar grendel with the 7.5 twist.

I'm of the mind set that I don't want 8 twist barrels any more.

And from now on 7.5 or 7 twist is where I want to be at.

Buenos side effect is with the tighter twist rate you rpm on your bullet goes up and that directly translates too bullet performance on game.

Is it measurable difference? Sure not by eye I'm sure. But my gut tells me a bullet that's spinning at 300,000 rpm is going to do more damage to a animal then a bullet spinning at 200,000 rpm.

Hope this helps OP.

[Lemonaid]

Good vid, thx for posting.

I think the point he was trying to make was accuracy issues at distance are more likely environmentally caused than by the transonic transition.

SY

That's what I got from it as well, meaning also (my first thinking) that when using a ballistic calculator you can mostly ignore the perceived limit where the bullet goes subsonic. A slower bullet with a higher B.C. should be a better contender for extended long range as wind will be the major factor causing misses if you get the drop figured out.

[Lemonaid]

LRRPF52 has stated many times that a tight twist will help keep the bullet more stable I.E well behaved as it transitions from supersonic to transonic.

After shooting 1000 yards with both at and bolt action grendels with 8 twist barrels and then getting to try it with my 16 inch ar grendel with the 7.5 twist.

I'm of the mind set that I don't want 8 twist barrels any more.

And from now on 7.5 or 7 twist is where I want to be at.

Buenos side effect is with the tighter twist rate you rpm on your bullet goes up and that directly translates too bullet performance on game.

Is it measurable difference? Sure not by eye I'm sure. But my gut tells me a bullet that's spinning at 300,000 rpm is going to do more damage to a animal then a bullet spinning at 200,000 rpm.

Hope this helps OP.

Dovetails to what I was thinking as well, or a more like for me an under educated guess!
One time I was trying to figure out why the loading manuals didn't include rotational energy with the calculation for kinetic energy. If I remember correctly it turns out to be a very small percentage that can be practically ignored.

[LRRPF52]

Where I noticed this, and it had already been mentioned by others pushing the .308 Winchester, was that the looser 1/12" twist just doesn't seem to hold together well once we started getting to 800yds and farther.

I can take the same bullet/load through another .308 with a tighter twist, and get predictable impacts as opposed to the wild impact beaten zone of the loose twist .308 Win.

Not only that, but with a tighter twist in a shorter barrel, I can get a tighter cone/groupings than a longer barrel/looser twist shooting the same bullet.

I've seen this with SCAR-H .308s that come through my DM or LR courses, but in the same conditions, tighter twist .308s can stack the rounds on a very small segment of a plate at 800-1000yds, normally the 175gr SMK or 178gr Hornady.

My 1/8 twist Liljas are really accurate though at distance, even from my lightweight Wasp contour 17.6" barrel Grendel.

[Lemonaid]

Thanks for chiming in LRRPF52! Appreciate the knowledge transfusion.

[Lemonaid]

What do you think of the twist rate stability calculators? Do they have a flaw that at some point the data they generate dose not match what happens in the real world?
I have been using the Berger and the JBM calculators and thought "I need to check on the stability at the velocity down range, like at 1000 yards and see if the bullet is still in the stable zone"
So I used data for the Sierra 130 game changer at different velocities at my altitude of 4500. Turns out that a 1 in 8 twist barrel will stabilize the GC at around a muzzle velocity of 1500 fps using the Berger calculator, the JBM was green down to 600 fps for muzzle velocity.
Anyone have a better method or calculator for determining bullet stability for the entire trajectory?

[Fess]

I think that the video has a confusing message. Perhaps he is reacting to those who say that all bullets will start tumbling immediately after hitting the transonic zone. Most bullets don't, but what actually happens is complicated and affected by the shape of the bullet and the spin in addition to atmospheric conditions. Yes, crosswind and other atmospheric conditions make it difficult to hit small targets at extreme range, but the presenter notes that accuracy tends to get significantly worse as the bullet pass the transonic region. He does not go into much detail, but acknowledges that some bullet shapes are less effected than others by the change to subsonic flight. The round nose 45-70 he noted is a good example, but there are current designs, like the Berger Juggernaut, that are specifically designed to stay accurate as they transition to subsonic speed.

As noted by several people already, spinning the bullet faster generally makes its trajectory more consistent as it approaches the transonic region. Today's high-quality bullets can tolerate the greater spin rate and provide consistent accuracy. This is helped by a good chamber and throat design that make certain that the bullets are correctly lined up as they pass through the barrel.

One of the main reasons the rifling twist has historically remained slow has to do with the relatively poor quality of bullets in the past. There are several issues with bullet strength and construction, but an interesting one has to do with bullet balance. A bullet that is not perfectly balanced along its long axis will try to spin off center as it travels down the barrel. The barrel will mostly keep that from happening, but as soon as the bullet leaves the muzzle, it will tend to jump to one side, affecting accuracy. It is even possible for an unbalanced bullet to maintain a stable wobble as it passes through the air, flying with higher drag and again, making the trajectory inconsistent from bullet to bullet. Over a hundred years ago, Dr. Mann (inventor of the "Mann barrel" used for accuracy testing) deliberately unbalanced bullets to see this effect. The point of impact varied with the orientation of the heavier section of the bullet as it left the muzzle. Slow twists helped minimize the effects of unbalanced bullets on accuracy.

[Wethepeople]

True, faster twist barrel shooting the same bullet gives a higher stability factor.
Not true. Higher BC bullets have a higher stability factor.
Example. A 180gr .308 Sierra GK has a G7 BC average of .242 between 3000-1500 fps.
A 180gr .308 Nosler Accubond has a G7 BC average of .246 between 3000-1500 fps.
Comparing each bullet from the same 1:10 twist barrel the GK has a higher stability factor. Both bullets have the same sectional density. The reason for this is ogive radius expressed in calibers, frictional surface area and boattail length and angle. In this case the lower average BC Bullet has a higher BC as it enters the transonic zone than the higher average BC bullet.
You can clearly see this in Bryan Litz book on Applied Ballistics for long range shooting.

[Wethepeople]

Interestingly, a bullet can be over spun. I'm sure many have heard the tales of jackets coming apart on light bullets in tight twist barrels. That has more to do with poor bullet design and or process controls than barrel twist rate. Some slower twist barrels I have shoot lighter bullet weight factory loads more accurately than my tighter twist barrels and vice versa. Again, that seems more dependent on charge weights and velocities rather than twist rate.

[LRRPF52]

This discussion is starting to get into some of the more advanced aspects of aerodynamics and will be relevant for the long range shooter if they are looking at controlling the types of bullets they use for 800yds and farther.

If you've ever watched the mach wave form on a supersonic object, whether it be an aircraft or bullet, there are certain behaviors that are very interesting. Aircraft meant for high supersonic speeds around or over Mach 2 (Think F-111, SR-71, MiG-25, F-22) will have different shaping compared to aircraft designed for subsonic or low-mid supersonic speeds (F/A-18).

Ogive shaping science dates back to artillery projectiles, rifle bullets, and really took off with the supersonic jet age and the Sears-Haack body theory. With early supersonic jet testing, they learned the hard way that aerodynamic shapes that work well in subsonic flight become useless or dangerous in supersonic flight, elevators being one example.



Relevant to bullets, if you look at fighter aircraft radome shapes, those aren't some designer just sitting there drawing what he thinks would be cool-looking. They are very math-heavy and are derived from a series of calculations which usually begin with what size the fighter's radar antenna needs to be in order to meet the detection and tracking range requirements set forth in the program specs. Once they determine the antenna size and gimbal limits (for mechanically-steered arrays, now being phased-out by electronically-steered arrays that don't physically move), they know the diameter of the radome seam with the forward fuselage and then can start playing with frontal Sears-Haack equations to determine the radome shape for the max speed requirements.



This will eventually lead to driving the aircraft weight, center of gravity (C of G), and center of aerodynamic pressure. With an aircraft, these aerodynamic centers can be shifted and are frequently in flight, especially with considerable changes in speed and fuel management.

With a bullet, one of the main differences is that it is gyroscopically stabilized to the tune of usually over 200,000rpm from centerfire rifles.

So now we have a spinning aerodynamic gryo that rapidly decelerates due to atmospheric drag and gravity negatively affecting its trajectory.

When it leaves the muzzle at its maximum velocity, there is a certain supersonic waveform being pushed by the bullet in the thick atmosphere, which provides significant drag on that bullet.



If the bullet would stay at the same speed through a uniform atmospheric density, the drag waveform would remain at a constant cone angle, but it of course doesn't due to deceleration.

As it rapidly decelerates, that mach cone waveform goes from being more elongated, to more flat, which affects the bullet's center of pressure.

If you can keep a VLD-shaped bullet (one with a long, secant ogive and long Sears-Haack-like boat tail) pointed optimally throughout the flight profile, it will retain energy better than if the nose deteriorates into an out-of-alignment gyroscopic wobble that bites into the radius of the mach cone, instead of staying oriented to the point of the mach cone.

Designing a bullet that will nose-over after the maximum ordinate rather than staying oriented to its original angle of attack (the difference in where the nose is pointed vs where the bullet is actually traveling) is one of the things Litz and Berger have been chasing, although much of this was pioneered by Swedes and Germans way back in the day for artillery.

At some of the Sniper Competitions I've RO'd or competed in, I had the opportunity to go pull TGTs that were well over 1100m, made from aluminum plates. We could see several projectile impacts that were still going supersonic at that range, but the bullets hadn't nosed-over, so you could see keyhole-shaped impacts that were all nose-up, as opposed to erratic. These were from either 7mm or .338LM or both, and the shooters were getting consistent and predictable hits at those ranges with the bullets coming down after the maximum ordinate. My guess is that due to the sheer mass of those projectiles, they were able to maintain enough inertia to stay supersonic, even with the additional drag from an AOA that appeared to be at least 25˚ or more, but they certainly were decelerating more than they would have had they been able to nose-over.

With the traditional rifles that start to peter-out at around 800yds using more tangent ogive bullets and token boat tails that don't do much for Sears-Haack Effect, my guess is that the increased frontal area of those bullets in a shorter amount of length, without the benefit of a long secant ogive cutting through the air ahead of the shank, causes them to deviate from their intended flight path more rapidly.

This is just a guess. Combine that with looser twists that don't maximize gyro stability, and I think this is why I have seen the beaten zone expand dramatically past 800yds, even up at 6600ft elevation. This effect has been noticeable with rifles that had no problem making consistent and predictable hits out to 800yds, then not even be able to get more than maybe 1/10 hits at 850yds, with the 1 being more luck, and impacts 3-4 mils left and right of the TGT.

[Kswhitetails]

Thinking about your comments on the tangent Ogives. Do you mean that the increased frontal drag results in an air brake effect and moves the center of pressure forward, creating the wobble earlier in the arc? The Secant bullets have more weight further toward the rear, which would help keep the CoP more rearward allowing more consistent flight characteristic further along the arc path. This would also lend my uneducated brain to conclude that they are also more likely to nose up beyond the maximum ordinate, but that's just a theory. You've given me more to go read about. Thanks a lot...

[Lemonaid]

Not on topic but interesting link to effects of rain on bullet trajectory.

https://www.gunsandammo.com/editorial/effects-rain-bullet-trajectory/372027

More on topic was a recent Guns and Ammo tv segment on flat based bullets vs. boat tail. I could not find a link so it may not be on their site yet.
It showed high speed video of both flat based bullets and boat tail bullets coming from the muzzle blast shock wave. Flat based rode ahead of it and boat tail had to go through it. That was accounted to as why flat based bullets were more accurate at up to around 400 yards.
At any rate I was trying to imagine was what happens when the rear shock wave hits the back end of the bullet at the transonic transition and if the flat based bullet would fare better or worse than the boat tail ?

[CJW]

All of you are down that damn rabbit hole so deeply you might not ever come back up!!!