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October 1st, 2015

The Transonic Zone — What Happens to Bullet Stability and BC


These four photos show the substantial changes in the shock wave and turbulence patterns for the same 7.5mm bullet at different velocities. The “M” stands for Mach and the numerical value represents the velocity of the bullet relative to the speed of sound at the time of the shot. Photos by Beat Kneubuehl.

“Going transonic” is generally not a good thing for bullets. The bullet can lose stability as it enters the transonic zone. It can also become less slippery, losing BC as a consequence of dynamic instability. In this video, Bryan Litz of Applied Ballistics analyzes what happens to bullet stability (and BC) as projectiles approach the speed of sound. Transonic effects come into play starting about Mach 1.2, as the bullet drops below 1340 fps.

Transonic Ballistics Effects Explained by Bryan Litz
What happens when the bullet slows to transonic speed, i.e. when the bullet slows to about 1340 feet per second? It is getting close to the speed of sound, close to the sound barrier. That is a bad place to fly for anything. In particular, for bullets that are spin-stabilized, what the sound barrier does to a bullet (as it flies near Mach 1) is that it has a de-stabilizing effect. The center of pressure moves forward, and the over-turning moment on the bullet gets greater. You must then ask: “Is your bullet going to have enough gyroscopic stability to overcome the increasing dynamic instability that’s experienced at transonic speed?”

Some bullets do this better than others. Typically bullets that are shorter and have shallow boat-tail angles will track better through the transonic range. On the contrary, bullets that are longer… can experience a greater range of pitching and yawing in the transonic range that will depress their ballistic coefficients at that speed to greater or lesser extents depending on the exact conditions of the day. That makes it very hard to predict your trajectory for bullets like that through that speed range.

When you look at transonic effects on stability, you’re looking at reasons to maybe have a super-fast twist rate to stabilize your bullets, because you’re actually getting better performance — you’re getting less drag and more BC from your bullets if they are spinning with a more rigid axis through the transonic flight range because they’ll be experiencing less pitching and yawing in their flight.

Bryan Litz Applied Ballistics Transonic Twist Rate Stability

To determine how bullets perform in the “transonic zone”, Bryan did a lot of testing with multiple barrels and various twist rates, comparing how bullets act at supersonic AND transonic velocities. Bryan looked at the effect of twist rates on the bullets’ Ballistic Coefficient (BC). His tests revealed how BC degrades in the transonic zone due to pitching and yawing. Bryan also studied how precision (group size) and muzzle velocity were affected by twist rates. You may be surprised by the results (which showed that precision did not suffer much with faster barrel twist rates). The results of this extensive research are found in Bryan’s book Modern Advancements in Long Range Shooting.

Bryan notes: “A lot of gunpowder was burned to get these results and it’s all published in layman’s terms that are easy to understand”. If you’re interested in learning more about transonic bullet stability, you may want to pick up a copy of Bryan’s book.

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September 22nd, 2014

Practical Thoughts About Transonic Bullet Stability and Accuracy

One of our Forum Members has a .308 Win load that dips into the transonic speed range at 1000 yards. He is concerned that his bullets may lose accuracy as they slow to transonic speeds: “My target is at 1000 yards. How important to accuracy is it to keep the bullet supersonic (Mach 1.2) all the way to the target? How does slowing to transonic speeds in the last 100 yards or so affect accuracy?”

TargetShooter Magazine and AccurateShooter.com contributor Laurie Holland offers some practical answers to this important question, based on his his experience with .223 and .308-caliber bullets.

Thoughts on Accuracy and Transonic Bullet Speeds by Laurie Davidson.
There is no simple answer to the question “How do transonic speeds affect accuracy”. Some bullets manage OK, some not so well, some fail entirely, and I’ve never seen a guide as to which models do and which don’t. But we do have the ‘boat-tail angle rule’, anyway. Bryan Litz says the ideal boat-tail angle is 7-9°. Go much above 10° and it’s too steep for the air to follow the bullet sides around to the base. This seems to manifest itself as much increased drag and turbulence leading to instability in transonic flight.

It is this effect that has led to the common advice of “Don’t use 168gr 30-caliber bullets at 1000 yards”. That is misleading advice as it resulted from use of the 168gr Sierra ‘International’ (aka MatchKing) bullet with its 13-deg BT angle. (This was, originally, a specialized 300m design — there are various near copies on the market from Speer, Hornady and Nosler.) By contrast, Berger 168-grainers are designed as long-range bullets with 8.9, 8.5 and a really nice 7° angle on the BT, VLD and Hybrid respectively. Hornady A-Max 30-cal projectiles (other than the 208-grainer) fall into this enforced shorter-range bracket too thanks to their 12.6° (and greater) boat-tail angles. (155gr = 13.5°, 168gr = 12.87°, and 178gr = 12.6°.)

Supersonic bullet shadowgraph

Even this boat-tail angle ‘rule’ doesn’t always seem to apply. Many older long-range Service Rifle shooters talk about good results at 1000 yards with some batches of 7.62mm match ammo in their 20″-barreled M14s using the 168gn SMK. I’ve successfully used Hornady and Sierra 168s at 1000 yards in 30-cal magnums which drive the bullets fast enough to keep out of trouble at this distance. This is still not recommended of course thanks to their low BCs compared to better long-range speciality bullets.

Supersonic bullet shadowgraph 7.5mm Swiss
These four photos show the substantial changes in the shock ware and turbulence patterns for the same bullet at different velocities. The “M” stands for Mach and the numerical value represents the velocity of the bullet relative to the speed of sound at the time of the shot. Photos by Beat Kneubuehl.

Transonic Issues with .223 Rem in F-TR
I was much exercised by [concerns about transonic instability] in the early days of F-Class, when I was shooting a .223 Rem with 80-grainers at 2,800 fps MV or even a bit less. Even the optimistic G1 ballistic charts of the time said they’d be subsonic at 1000 yards. (Bryan Litz’s Point Mass Ballistic Solver 2.0’s program says 1,078 fps at 1000 yards at 2,800 fps MV in standard conditions for the SMK; below 1.2 MACH beyond a point somewhere around 780 yards.) In fact they shot fine in a large range of conditions apart from needing around 60% more windage allowance than 6.5mm projectiles [shot with a larger cartridge]. The biggest problem apart from my wind-reading skills was constantly getting out of the rhythm to call to have the target pulled as the pits crew didn’t hear the subsonic bullets and had trouble seeing their little holes.

In the early days of F-TR I used a 24″ barrel factory tactical rifle that was billed as F-TR ready — it wasn’t! The much touted 175gr Sierra MatchKing, as used in the US military M118LR sniper round, was allegedly good at 1000 yards at .308 velocities — but it wasn’t! It would group OK in [some calm] conditions, but any significant change would cause a much greater deflection on the target than the ballistic charts predicted, so transonic flight was obviously making it barely stable. I also suspect conditions on the day had a big effect as Litz’s program says [the 175gr SMK] is just subsonic at 2,650 fps MV at 1,000 in standard conditions. Throw in MV spread and there was a risk of some round remaining supersonic, while others went transonic. Plus warmer or colder air moving onto the range under some conditions might change things.

I used the combination on Scotland’s notorious Blair Atholl range at 1000 yards in one competition in a day of cold headwinds from the north and frequent rain squalls. The temperatures plummeted during the squalls (and the wind went mad too!) and what was an ‘interesting group pattern’ outside of squall conditions changed to seeing me do well to just stay on the target frame at all. On ranges other than Blair (which is electronic, so no pits crew), target markers reported they heard faint supersonic ‘crack’ and saw round holes on the paper, so the bullets appeared to remain stable and just supersonic in summer shooting conditions.

Transonic Problems with M118LR 7.62×51 Ammo
Confirmation of this transonic performance phenomenon has since come from USMC snipers who say the M118LR’s performance ‘falls off a cliff’ beyond 800m (875 yards), which is just what I found when shooting the bullet at slightly higher than M118LR muzzle velocities. A move to the 190gr SMK with Vihtavuori N550 keeping the MVs reasonable gave a vast improvement in 1000-yard performance.

Practical Advice — Use a Bullet That Stays Supersonic
The ‘easy’ / better answer to all this is to use a design such as the 30-caliber, 185gr Berger LRBT with a reputation for good long range performance and to load it to achieve or exceed 1,350 fps at 1000 yards. If I can get the combination I’m using to be predicted to hold 1,400 fps at this range in a G7-based program calculation, I’m happier still.

Incidentally, the old long-range, 30-cal Sierra bullets (the venerable 190gr, 200gr, and 220gr MatchKings), with their extra length boat-tail sections, have a superb reputation for stable transonic / subsonic flight. They were used by GB and British Commonwealth ‘Match Rifle’ shooters at 1000, 1100, and 1200 yards for many years before the current bunch of 210gr and up VLDs and Hybrids appeared.

Supersonic space shuttle waveTransonic vs. Supersonic

The term “Transonic” refers to velocities in the range of Mach 0.8 to 1.0, i.e. 600–768 mph. It is formally defined as the range of speeds between the critical Mach number, when some parts of the airflow are supersonic, and a higher speed, typically near Mach 1.2, when the vast majority of the airflow is supersonic. Instability can occur at transonic speeds. Shock waves move through the air at the speed of sound. When an aircraft goes transonic and approaches the speed of sound, these shock waves build up in front of it to form a single, very large shock wave. This is dramatically illustrated in this Space Shuttle photo.

Permalink Bullets, Brass, Ammo, Tech Tip 6 Comments »
April 1st, 2013

Berger Bullets Introduces Revolutionary Sonic Ripple Bullet

At the end of this year, Berger Bullets plans to introduce a new projectile that may truly be the most revolutionary bullet design since the advent of jacketed spitzers in the late 19th Century. Berger’s new bullet is unlike anything we have ever seen before. It features concentric curved ridges, or “ripples”, on the bearing surface. Tests show that this new projectile, dubbed the “Sonic Ripple Bullet”, has signficantly less drag than conventional bullets (no matter what their ogive configuration). In addition, the Sonic Ripple design provides increased stability at all velocities (allowing barrels with slower twist rates for a given bullet weight).

Berger Bullets Revolutionary Sonic Ripple Bullet

So, what does all this mean in practical terms? Well, compared to conventional bullets (of similar weight/size), the Sonic Ripple Bullet will shoot a flatter trajectory, buck the wind better, retain energy longer, and remain stable for a much longer distance. That’s big news for competitive shooters, tactical shooters, and long-range hunters.

Berger Bullets Revolutionary Sonic Ripple Bullet

Berger Bullets Revolutionary Sonic Ripple BulletThe Science of the Sonic Ripple Bullet Design
Bryan Litz, Ballistician for Berger Bullets, explains: “This radical leap forward in bullet design was made possible by advanced, new bullet-making technologies. The unusual bullet appearance is only part of the revolutionary ‘Sonic Ripple’ system. The curvilinear waves or ripples in the bullet jacket are designed to create a specific resonance when fired from a specially ‘tuned’ barrel system. The result is an optimization of the sonic wavefront created by the bullet as it travels through its trajectory. This wavefront optimization simultaneously reduces bullet drag while increasing bullet stability.”

In essence, the supersonic shock-wave is smoothed out, dramatically reducing secondary wave fronts. This is all good, as Bryan explains: “If all the internal ballistic requirements are met, the Sonic Ripple bullet exits the muzzle with a harmonically-stabilized launch dynamic. As a further benefit of the ripple design, tests show that the concentric ripples also enhance boundary layer airflow attachment on the bullet. This, in turn, dramatically reduces wake turbulence and attendant drag.”

The reduction of wake turbulence (combined with wavefront optimization) represents a “major breakthrough” which should increase projectile BC by at least 0.14 (on G7 scale), according to Bryan. But, we wondered, might the increased surface area associated with the ripples slow the bullet down in flight? Actually, no. Bryan explained: “Eddies in the boundary layer around the ripples actually lower skin friction drag which more than compensates for increased surface area, resulting in a net friction drag loss at all velocities — both supersonic and transonic.”

Berger Bullets Revolutionary Sonic Ripple Bullet

Sonic Ripple Bullets Available by the End of 2013
When will we see Sonic Ripple Bullets on dealers’ shelves? Maybe this year. Berger’s marketing department told us: “The Sonic Ripple technology is currently under development and is expected to mature enough for commercial application by late fall, 2013.”

Permalink Bullets, Brass, Ammo, New Product 25 Comments »