Has anyone fired projectiles at slower speeds to help determine the accuracy at longer distances?

[Healthservices]

This is messing with my head ever since I have thought it up and not tried it. What else am I missing? Other than a couple of marbles?

Please correct me if I am wrong, I am still learning myself. Just because a projectile will do MOA at 200 yards, will it mean it will continue to do so out to longer distances? No? how can we determine how well it will do beyond that point without wasting a ton of amo?

And yes there are other factors such as wind and air density etc, but I do believe how well a projectile flies at lower speeds is just as important as how well it does at higher speeds. Say at 200 yards you are 1/2 of your way to 400 yards. By plugging into a balistic program we can get an idea of what speed the projectile will be at 200, 300, and or any given point.

If this gives the speed of how fast the projectile is at any given point. And I wanted to see how well a projectile would do to say at 400 yards and can only shoot to 200 yards at my local range, (actually I dont have access in orange county to more than 100) would shooting the projectile at the speed it is at 200 yards from my position to 200 yards give me a good idea of how it would shoot from 200 to 400 yards? For example in the graph below

If we shoot a projectile at a target at 200 yards with a inital speed of 1040 fps and find out it is sub moa projectile to 200 yards, can we now shoot the same projectile using the graph above for that projectile at 810 fps to a 200 yard target to predict how well it will fire to 400?

Would this work and what factors would cause it not to work?

Other than maybe the ability to fire the same projectile from the same gun at lower speeds.

Allen

 

[karl_h]

No.

Would this work ...

No.

 

[Healthservices]

So what would be the reason/reasons why not?

Allen

 

[JaceSpace1369]

So what would be the reason/reasons why not?

Allen

I would assume it wouldn't work when shooting slugs due to the slugs needing sufficient speed to stabilize first.

On the other hand, I don't know if speed reduction destabilize the slug after it's already been stabilized. Obviously BC drops the further you go as it's speed reduces, but I am unsure.

 

[SilentSquirrel]

No.

No.

Explain, please!

 

[Healthservices]

Dang, oh well. I was just hoping. And thinking back when i was trying to fire the 40 gr javelins out of a shorter barrel and not achieving any thing considered high speed they did horribly on groups

Allen

 

[JaceSpace1369]

Dang, oh well. I was just hoping. And thinking back when i was trying to fire the 40 gr javelins out of a shorter barrel and not achieving any thing considered high speed they did horribly on groups

Allen

I believe that shooting slugs at slower speeds would be a good way to judge if the slug will shoot well at higher speeds. I have been able to shoot slug brands such as NSA, ZAN, and Varmint Knockers as slow as 770fps with good success and accuracy only improving the faster I shot them.

The other thing with shooting at 400yds and testing only to 200yds with a slower velocity even if stabilized is that you won't get the same conditions across 200yds as you would across 400yds.

 

[Healthservices]

I believe that shooting slugs at slower speeds would be a good way to judge if the slug will shoot well at higher speeds. I have been able to shoot slug brands such as NSA, ZAN, and Varmint Knockers as slow as 770fps with good success and accuracy only improving the faster I shot them.

The other thing with shooting at 400yds and testing only to 200yds with a slower velocity even if stabilized is that you won't get the same conditions across 200yds as you would across 400yds.

Wait, see that's the part that gets me. You were able to shoot those slugs at lower speeds so you know when they slow down to those speeds down the range they will still be good. Also meaning you know at what speeds (or window of speed) they stay on target.


Allen

 

[Healthservices]

When I shot the javelins at slow speeds that one time and they did not group, I did not think much of it but when I was able to up the speed I was able to do fairly well to 200 yards but beyond that it was not so good. Well actually I don't know where it starts going bad as the next set of targets was at 300 yards and it would not do that, at least for me it didn't

Allen

 

[SilentSquirrel]

Wait, see that's the part that gets me. You were able to shoot those slugs at lower speeds so you know when they slow down to those speeds down the range they will still be good. Also meaning you know at what speeds (or window of speed) they stay on target.


Allen

As much as there could be variables that can't be duplicated by shooting at lowered velocity
(Such as how muzzle velocity & twist rate work together stabilizing the projectile)
I think you have the basis for a great experiment, Allen

And obviously the answer is not "No."

Go for it!

Edward

 

[JaceSpace1369]

Wait, see that's the part that gets me. You were able to shoot those slugs at lower speeds so you know when they slow down to those speeds down the range they will still be good. Also meaning you know at what speeds (or window of speed) they stay on target.


Allen

I think for what you are looking for, it may be a good way to find out. Shoot to 100yds with your max preferred velocity, calculate the bc, then shoot to 200 and calculate the BC at that distance. Use those numbers to get an approximate BC/speed for 300 and 400yds and shoot those speeds at 100yds and see how it goes.

 

[Ballisticboy]

Unfortunately, shooting the projectiles at lower speeds from the same gun will not reproduce how it will fly at long ranges due to the different rates of loss of spin and speed. To reproduce the projectile flight down range from the gun, you would need a barrel with a higher twist rate, which can also give the same yaw rates as the projectile experiences as it slows down. This problem has been known in small arms testing circles for many years.

 

[Healthservices]

Unfortunately, shooting the projectiles at lower speeds from the same gun will not reproduce how it will fly at long ranges due to the different rates of loss of spin and speed. To reproduce the projectile flight down range from the gun, you would need a barrel with a higher twist rate, which can also give the same yaw rates as the projectile experiences as it slows down. This problem has been known in small arms testing circles for many years.

I kind of thought there was something else like loss of spin, speed I know is going to be loss. Can loss of spin be calculated?

Allen

 

[Healthservices]

It spins faster down range? Oh wait I understand higher meaning slower maybe a regular superior liner with a higher twist?


Allen

 

[Berserkeley Mike]

A typical airgun barrel has a 1 in 16 twist, 1 turn in 16" or in 1.33 feet.
At 1040 fps out the barrel a projectile is spinning at 1040 / 1.33 = 780 rps, revolutions per second. When it slows to 810fps it is still spinning close to 780 rps.
If the pellet started out the barrel at 810 fps it would be spinning at 810 / 1.33 = 608 rps.
What that means for accuracy, I don't know.

 

[Ballisticboy]

I kind of thought there was something else like loss of spin, speed I know is going to be loss. Can loss of spin be calculated?

Allen

It can if you know the spin damping coefficient. I only have it for one pellet design, though it will not be hugely different for other pellets unless they have some special features which change it. It has been measured at different ranges before now by firing a pellet through two sheets of paper a few inches apart at different ranges. The pellet had an ink mark or something similar that marked the paper as it went through, thus allowing the twist rate to be measured and calculated. It was from that data that I was able to calculate spin damping coefficients.

 

[Bedrock Bob]

The short answer is no. You can't determine accuracy at longer distances by shooting a projectile slower at shorter distances.

You may extrapolate some good info on trajectories and speed relative to pressure. But not accuracy at range. Accuracy at longer ranges depend a lot on the balance and symmetry of the projectile and external factors like wind. At shorter ranges that may matter very little.

A projectile that rotates a bit off its CG may shoot 1 moa at 100 yards but 6 moa at 300. A more balanced, longer projectile may hold MOA accuracy at much longer distances.

Depending on barrel twist, projectile length and velocity a slug may not stabilize at shorter ranges yet perform well out far. Lots of projectiles need some yardage to spin on center. They may not shoot the best at shorter ranges where projectile stability matters less.

Errors in stability multiply themselves downrange. A whole lot more is going on besides velocity decay and trajectory comparisons.

You can plot a drop and velocity curve and get an educated guess at what may happen downrange. Its a worthy analysis. But once you shoot at distance things may not add up as you expect.

 

[Berserkeley Mike]

There is the story of the Experimentalist and the Theoretician, both on death row facing execution...
Ballisticboy is the experimentalist! I'd be interested in his actual measurment of spin damping.

Anyway, as I understand it, spin is damped in flight by air resistance or drag across the circumference of the pellet/slug, similar to what slows the pellet in its flight path but in the roundabout direction. We can calculate the approximate rotational velocity of the surface of the pellet/slug when it leaves the barrel, and compare that with the flight velocity. We know how much the flight is slowed by drag, and we know that drag increases with the square of velocity, so if the rotational velocity is similar to the flight velocity, it might slow down similarly, and if it is less then it slows down much less.
Let's take those Javelin 40gr slugs as an example. They are 0.217" in diameter. Circumference is pi*diameter, so 3.14 * 0.217 = 0.68" around. If it was spinning at 780rps when it left the barrel at 1040fps, then the surface would be spinning at 0.68 inches per revolution * 780 revolutions per second = 532 inches per second or around 44 feet per second. That also can be calculated (as a confirmation) by the twist rate, 1 in 16, where 1040 feet per second times 12 inches per foot / 16 inches per twist gives 780 twists per second or revolutions per second. And then it fits into the calculations preceeding to give the 44fps.
That all is very slow relative to the flight speed. I wouldn't expect the rotation to slow much as drag is low at that speed.

Mike

 

[Berserkeley Mike]

Ted Bier has a video that might be clarifying aboyt accuracy and velocity. Note that at higher speed there was spiralling:

Matt Dubber then explains what might be happening:

In my understanding, the key concept is that rotation speed increases with pellet speed, but rotation doesn't decrease with the speed decrease one sees with distance. So a fast shot pellet may be over-spinning at distance, while a slow shot pellet is still spinning in a stable manner at distance, even if it is dropping like a rock at such distance.

 

[Ballisticboy]

There is the story of the Experimentalist and the Theoretician, both on death row facing execution...
Ballisticboy is the experimentalist! I'd be interested in his actual measurment of spin damping.

Anyway, as I understand it, spin is damped in flight by air resistance or drag across the circumference of the pellet/slug, similar to what slows the pellet in its flight path but in the roundabout direction. We can calculate the approximate rotational velocity of the surface of the pellet/slug when it leaves the barrel, and compare that with the flight velocity. We know how much the flight is slowed by drag, and we know that drag increases with the square of velocity, so if the rotational velocity is similar to the flight velocity, it might slow down similarly, and if it is less then it slows down much less.
Let's take those Javelin 40gr slugs as an example. They are 0.217" in diameter. Circumference is pi*diameter, so 3.14 * 0.217 = 0.68" around. If it was spinning at 780rps when it left the barrel at 1040fps, then the surface would be spinning at 0.68 inches per revolution * 780 revolutions per second = 532 inches per second or around 44 feet per second. That also can be calculated (as a confirmation) by the twist rate, 1 in 16, where 1040 feet per second times 12 inches per foot / 16 inches per twist gives 780 twists per second or revolutions per second. And then it fits into the calculations preceeding to give the 44fps.
That all is very slow relative to the flight speed. I wouldn't expect the rotation to slow much as drag is low at that speed.

Mike

It was Harry Fuller in Australia who measured the down range spin rates using the method I described earlier. He used 0.25 JSB King pellets fired out to 200 yards at 890 ft/sec. I just used his data to obtain the spin damping. Spin damping moment coefficients are very small but so is the spin inertia of a pellet

As for Matt Dubber, this is one of his better ballistics videos. He starts out on the right track, but then looses his way somewhat in getting hung up on crosswinds, and missing the root cause of the spirals. Crosswinds can induce spirals, but they are not the basic reason pellets spiral. The actual problem appears to be the increase in yaw wave lengths, which increases the heave and swerve amplitude to the point where the pellet gets visible spiralling. The lack of dynamic stability of pellets doesn't help either, as it provides the increasing yaw angles which help to cause the heave and swerve.

Spin damping is related to the skin friction drag, not the total projectile drag. On a pellet the largest drag contribution is the base drag followed by the nose form drag at high speeds. The skin friction drag is a relatively small part of the total.