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°.)
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.
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.
 Transonic 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. |
Similar Posts:
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Tags: Laurie Holland, M118LR, MatchKing, Supersonic, Transonic
Good write up!
Lots of mystery surrounding transonic instability and how it limits range for some bullets.
One item not mentioned in the write up is the effect of altitude. At sea level, especially in cold weather, the list of bullets that are transonic stable is very short. On the other hand, get up to 5,000+ feet above sea level and almost all bullets are transonic stable due to reduced air density (plus they go a lot farther before slowing to transonic speed at high altitude).
-Bryan
I’ll second Bryans opinion that this is a great article. I’ve always wondered about the transonic nature of bullets and how much practical accuracy is lost.
A lot of variables. I don’t think bullet stability is easy to predict in transonic. Maybe not possible to predict.
The 80gr 224 bullet example, is a good example to illustrate. While the bullet would be unstable from the MUZZLE at 1078fps(even from 7.5in/turn), that’s not so at 1000yds. This, because bullet velocity slows way more than rotation, leading to ~3.3in/turn by 1078fps/1000yds. Sg would be around 9 there(very high).
So it’s a matter of dynamic stability, which is less than predictable, and demonstrated through field testing.
So in the end they shot good, with expected side affects of higher drag, and fixed distance.
Where this would suck is with multiple distance shooting(hunting/tactical).
I tried to explain this to a marine sniper on another forum. He just laughed and said he has pictures of steel targets that were shoot by .308 from 1600 yards away. I believed him unti he said the bullet impacts where proof the bullets weren’t yawing. I know a bullet will show if it’s yawing a lot but I don’t think you can tell a difference in slight yawing. I’ll trust those pictures over splats on steel. He’s the owner of SP
We have some data on BC measurements of a prototype .338 caliber bullet we have shot to 2960 yds. What we observed was a rise in BC at extreme range, and round bullet holes. This indicates stability of the bullet traversing the transonic zone. We estimated that the bullet under the conditions of the test and a 3000 fps MV went subsonic at about 2400 yds and was stable until it hit the target at 2961 yds.
Forgot to mention that the altitude we conducted our test at was 1600 ft