Subsonic and Accuracy

[bsekf]

What happens to a bullet when it goes subsonic? I keep reading where the 1000 + yard shooters want their bullet to stay supersonic, but why?????? Some say the lower velocities buck the wind better. I often find the best accuracy node well below the max velocity. Lower velocity has to save barrels. What happens when the velocity drops below 1200 FPS?

Bill

 

[#40Fan]

If you look at Sierra's BC ratings, they are nice enough to give you a BC for specific speeds. Other than some handgun bullets, most of them have a higher BC when pushed faster.

 

[Sniper338]

When the bullet goes subsonic it de stabilizes and starts going in accurate, at some point it starts tumbling.

 

[dcali]

Aerodynamic chaos happens near the sound barrier. This results in lots of weird behavior, including the potential for destabilization. It’s best avoided. And if you can’t, some bullets are better than others and making it through. Exactly what helps is a very complex topic.

 

[dgeesaman]

Aerodynamic chaos happens near the sound barrier. This results in lots of weird behavior, including the potential for destabilization. It’s best avoided. And if you can’t, some bullets are better than others and making it through. Exactly what helps is a very complex topic.

Another way to look at it is the sudden change of flow patterns between supersonic and subsonic. Not only is the bullet dynamically unstable, the change of flow pattern will push it offline. Once it gets to tumbling of course it's really going off line.

 

[Dusty Stevens]

 

[Texas10]

Here's the long version Bsekf.

As the bullet slows to transonic speeds, the super sonic shock wave that had been in front of the bullet moves back towards the heel. This moves the center of air pressure back and creates an aerodynamic overturning effort on the bullet that tends to make it behave like an arrow shot backwards (if that were possible), in effect wanting to turn it around backwards. Of course long before this happens, the bullet is zig zagging all over the place as it wobbles erratically. Although it has slowed down considerably since it left the barrel, it's spin rate has not slowed down nearly as much, and is still acting as a gyroscope to try to stabilize the bullet. And this is where things get really weird as an upward force applied to the bullet tip will be delayed for 90 degrees in the direction of rotation, causing the bullet to yaw right instead of up.

This new yaw creates a new aerodynamic effort against the bullet which is again delayed 90 degrees which causes the bullet to then yaw downward. This leads to an increasing wobble until the bullet is very often sideways as it goes through the target, and accuracy of course has long since gone right out the window.

So when shooting long distance, take into account elevation and temperature, both of which effect air density and the speed of sound, and therefore the velocity at which your bullet will begin to de-stabilize.

 

[rebel]

But if you start subsonic and end subsonic, the bullet can be very stable

 

[dcali]

To be clear, the sound barrrier does not automatically make a bullet unstable. It does tend to make aerodynamic effects less predictable - lots of hard to measure weirdness is going on. But you absolutely can shoot through the transonic.

 

[Outrider27]

Always remember what damoncali said; some bullets are definitely better at making it though than others; an extreme example is Cpl. Gregory Harrison's 2703 yard kills in the sandbox. Based on my calculations with a load I use that I'm pretty sure is close to what he used in his .338 Lapua, those 3 projectiles were well under the speed of sound at impact, so they made the transition well.

Note that we said 3 projectiles - most accounts leave out his 2nd and 3rd shots, the 3rd being even more amazing than the first two. He and his spotter got to try 6 to 8 shots at stuff in the general area to get sighted in as best as one can under those conditions, and the conditions were good - early morning and little breeze - then the gunner and assistant gunner showed up and set up their machine gun. First Harrison took out the gunner, the shot widely covered. Then the assistant gunner stepped up to take his place, and Harrison did it a second time. Then for good measure, the really spectacular shot, the one that seldom gets mentioned; at 2703 yards, he put one shot into the receiver of the machinegun, a much smaller target than either human, permanently disabling it. That load (possibly a 300gr projectile, more likely a 250gr projectile) passed through the sound barrier and went subsonic with decent and repeatable stability and accuracy, so going subsonic is not necessarily the kiss of death for accuracy at long ranges, just one more thing we have to deal with in searching for an accurate load at extreme (for the caliber) ranges.

Just a random thought....

 

[270WinDude]

In physics there is different math used to calculate things like jet airplanes and whatnot moving beyond the speed of sound then you’d use for subsonic if not mistaken. The term barrier makes sense I guess.

 

[dgeesaman]

In physics there is different math used to calculate things like jet airplanes and whatnot moving beyond the speed of sound then you’d use for subsonic if not mistaken. The term barrier makes sense I guess.

As an accelerating object approaches Mach 1, it builds up a layer of denser air in front of it caused by sound it’s basically catching up to. The longer you hang out just below Mach 1, the more intense this effect. Thus the term barrier. Back when Yeager was piloting the X-1, they learned that it mattered to accelerate aggressively through Mach 1.

David

 

[bsekf]

Thanks for the replys. I kept thinking of a aircraft going from supersonic to subsonic, they don't start to tumble. However maybe that transition is part of the reason the Concord isn't flying any more..... I never talked to a pilot or a Concord passenger to ask them what happpened when they dropped back through the sound barrier. But, a bullet isn't winged, so it may be completely different. That video is certainly impressive, but the plane was accerelatering through the sound barrier, not dropping back through it, like a bullet. Easy to understand punching through the air to supersonic, hard to understand why dropping back would be as unsettling. And, the Harrison shots adds even more confusion.

Bill

 

[dgeesaman]

Thanks for the replys. I kept thinking of a aircraft going from supersonic to subsonic, they don't start to tumble.

In fact, Yeager has special controls added to the X-1 because the craft got very unstable in the transonic regime. That learning experience allowed future aircraft designs to pass the sound barrier up and down safely.

David

 

[4xforfun]

Question...and maybe the answer to my question may help explain the transition from super to sub.

Why is it so hard to shoot small long range groups with subsonic loads? A while back a guy posted some video shooting a 500 yard target with a 300 wisper. Looked to me like 6 -10 FEET of vertical at 500 yards. I thought is was poor, but the response was..."try it with a subsonic load some time". The wind gets you. My response was that would give you horizontal, not vertical. I don't remember where it ended up.

Kind of put damper on my dream of a suppressed 338BR shooting 300's.

Thanks,
Tod

 

[KevinThomas]

If you look at Sierra's BC ratings, they are nice enough to give you a BC for specific speeds. Other than some handgun bullets, most of them have a higher BC when pushed faster.

That’s not entirely true. Sierra lists multiple BCs as a way of force-fitting thei bullets into a drag model (the G1) which isn’t suited to most of their bullets. Most of their line would be much better served bye the G7 model, which Berger and Lapua use for most of their match type bullets. The Sierra multiple BC bands were developed by Bill McDonald and Ted Almgren as a means of better matching the actual resulting trajectories to a drag model that they knew wasn’t a good fit. At that time, virtually everyone in the industry simply used ththe G1 as a universal default drag model, accepting the inherent inaccuracies of the resulting trajectory predictions. It was a good attempt to rectify a problem they were well aware of, without resorting to using the more appropriate G7 model. At that time (30 years or so ago) very few shooters had any understanding of BC as a general concept, much less the various drag models appropriate for a multitude of different bullet profiles.

Hope that helps a bit.

 

[Outrider27]

My 45 ACP has sub sonic, MV I would have to hold over about 35 ft at 500 yards.
God only knows how much wind deflection

Big bore handguns do tend to have huge mid-range trajectories, lol. Never shot mine beyond 200 meters (IHMSA big bore matches), and even the .44 Mag had a mid-range trajectory on the order of 4 feet at that range with a max load of 2400 pushing a 240gr bullet - the .45 ACP made that look puny. Much preferred my 7BR XP-100 or the 30-30 Contender, which had laser trajectories compared to that (high single digit inches).

But Cpl. Harrison was dealing with the same things; my calcs showed he hit max elevation at about 1600 yards into the 2703 yard flight, and that the distance above line of sight was a bit over 150 feet (!). That figure really got my attention....

 

[rebel]

Question...and maybe the answer to my question may help explain the transition from super to sub.

Why is it so hard to shoot small long range groups with subsonic loads? A while back a guy posted some video shooting a 500 yard target with a 300 wisper. Looked to me like 6 -10 FEET of vertical at 500 yards. I thought is was poor, but the response was..."try it with a subsonic load some time". The wind gets you. My response was that would give you horizontal, not vertical. I don't remember where it ended up.

Kind of put damper on my dream of a suppressed 338BR shooting 300's.

Thanks,
Tod

Because of the powders or reduced loads you need to shoot subsonic, you get ES's that can be very high. That results in your vertical. Once you have that as tight as you can get it, think of the trajectory - a 220 MK from a 300 Blk is around 18 inches high at 100 yards if zeroed at 200 yards. At 150 it's dropped a lot but not as much as it's going to in the next 50 yards. Instead of a rainbow, you have sort of quizzical eyebrow trajectory. Most rifles shooting these cartridges are not bench guns, therefore do not slide in the bags and return to zero. You have to aim in the same spot, in the same way every time. In other words, you have to work for it because the rifle isn't going to do it for you. Shooting at 200 yards with a heavy sub is akin to shooting at 1000 yards with a conventional super - except the sub never enters transonic, just starts falling out of the sky. Controlling that fall is tough.
I don't even want to talk about wind