Bullets and BC's... thought experiment

[DocUSMCRetired]

Spin relative to velocity is the big question. For the most part my answer would be the same Doc gave....that is, "spin rate decay is almost a non issue." However, this patented answer is because we do not really know what the spin rate is at the target (1,000, 1,500, or 3,500). The guesstimates are 25-60% ranges depending upon bullet diameter. Are we missing something here? Is there an actual way to determine if this is something to worry about? Probably never have to be concerned with the issue except for ELR, but would be nice to know.

We actually do know what the spin rate is at the target, devices are out there to measure this. We have done these kinds of tests in our lab.

 

[284winner]

I'm in the camp that the heavies win out with BC being equal simply because I've seen it with my own eyes a number of times. In my mind and looking at the ballistics as others have pointed out...all should be equal. We have had 25 shooters at the line and about half shooting small caliber (6mm/6.5mm) with ballistics that show the same drift as the other half shooting 7mm/30 cal. and upon wind switches at 600 yards virtually every small cal group would be moved over 1-2 inches further than the heavies. We have actually experimented with this and seen it a number of times. I do not have the answers but many of our shooters have witnessed this phenomenon. This thread specifically stated 1,000 yards plus! Again, as has been indicated we have many changing conditions and it becomes very hard to determine if some of this is BC or a couple of dozen other factors. I have asked the question on a couple of other threads about spin decay rates and not gotten responses. If we place any faith in the Kolbey spin decay rate formula, there are questions that come to mind. First off, solving for target spin rate (regardless of range), we only have 3 parameters to input, and that is bullet rpm at the muzzle, flight time, and bullet diameter. As an example if you calculate for 3 different calibers, .308, .338, .375 at 3,000 muzzle velocity which gives 240,000 rpm, we will see a pretty good difference in spin decay by bullet diameter. At 1800 yds the 308 loses around 27%, the 338 24%, and the 375 21%. As the distance goes further the decay gets worse but not near as bad as velocity. Regardless of the accuracy of this formula, is it possible that the decay rate of spin (whatever that is) and velocity at long ranges causes unusually long skinny bullets to become unstable although our calculator shows it to be stable. We have solid bullets with super high BC's and low weights but will not hang with the heavies of the same length at really long ranges (1-2 miles). Although a few 338's have been successful in the KO2M events, more often than not the heavies will be used at much lower velocities. I don't have any answers, just asking the question....could spin decay have anything to do with the original question?

I think you do have answers. What you stated above was pretty much my experience shooting heavy vs light (same BC) at beyond 1200 yards. Tough to deny experience in the field vs theory ( ballistic programs) when Ma nature comes to play. We're not seeing the 338/375s in the ELR Match's because the 195s from the big 7mm cartridges shoot as well with the same BCs at even high velocities. There's a reason for it. I don't have answers for this other than proof is at these ELR events.

 

[Longrange57]

We actually do know what the spin rate is at the target, devices are out there to measure this. We have done these kinds of tests in our lab.

Doc, is the info available and does it coincide with Kolbeys formula?

 

[Longrange57]

I think you do have answers. What you stated above was pretty much my experience shooting heavy vs light (same BC) at beyond 1200 yards. Tough to deny experience in the field vs theory ( ballistic programs) when Ma nature comes to play. We're not seeing the 338/375s in the ELR Match's because the 195s from the big 7mm cartridges shoot as well with the same BCs at even high velocities. There's a reason for it. I don't have answers for this other than proof is at these ELR events.

284, I follow you. Guys here have great points. I was not aware of devices for purchase to measure spin at distance. Was not aware that this testing was done as Doc indicated. Hope it is info we can obtain and make observations from. I'm trying to help a friend and both of us understand what may be going on at 2200 to 2500 with a .375 and muzzle vel of 2930. Shows to be stable, but occasionally hits would show otherwise. Regular hits at 2500 yds. At 2600-2700 it starts to go subsonic and that may be our whole issue.
I personally shoot a 6.5 x 284 at 1400 yds (.670 BC) and have a blast! However, a 300 WSM with Berger 210, BC of .621, will shoot inside of me all day long in the wind! LOL

 

[DocUSMCRetired]

Doc, is the info available and does it coincide with Kolbeys formula?

It is state of the art circa decades ago, and requires only materials from home depot on the cheap end of doing it. Just build a spin box, and shoot through it.

 

[Longrange57]

It is state of the art circa decades ago, and requires only materials from home depot on the cheap end of doing it. Just build a spin box, and shoot through it.

Sorry to bother you! I see that spin rate decay truly is a non issue....at least to Applied Ballistics!

 

[Ballisticboy]

While knowing the spin rate down range is nice and allows estimates of the spin damping coefficients, it does not help a great deal in assessing the stability of a bullet. The gyroscopic stability will increase as the bullet moves down range unless the bullet is a special with spin control devices. Trying to get a realistic estimate of the dynamic stability will still be extremely difficult and any modelling will be fraught with danger. I have managed to model dynamic instability using a six degree of freedom model but how close it is to reality is debatable.

We have used a doppler tracking radar to measure the down range spin rates of projectiles.

 

[DocUSMCRetired]

While knowing the spin rate down range is nice and allows estimates of the spin damping coefficients, it does not help a great deal in assessing the stability of a bullet. The gyroscopic stability will increase as the bullet moves down range unless the bullet is a special with spin control devices. Trying to get a realistic estimate of the dynamic stability will still be extremely difficult and any modelling will be fraught with danger. I have managed to model dynamic instability using a six degree of freedom model but how close it is to reality is debatable.

We have used a doppler tracking radar to measure the down range spin rates of projectiles.

This unfortunately applies to a number of variables, especially when it comes to 6DOf.

 

[Ballisticboy]

This unfortunately applies to a number of variables, especially when it comes to 6DOf.

This is why they should only ever be used by people who know what they are doing.

 

[JPeelen]

It is state of the art circa decades ago, and requires only materials from home depot on the cheap end of doing it. Just build a spin box, and shoot through it.

Alas, an Internet search for spin box is heavily contaminated by the computer user interface element of the same name.
Can you point us to a good source describing how a spin box works ? Thanks.

 

[dcali]

For a quick demo of how difficult some of this stuff is, you can try out Lapua's 6DOF phone app. There is a feature where it shows a chart of Sd vs Sg. I plugged in some typical .338 loads, and found that it predicts dynamic instability at unrealistically short ranges. That feature is basically worthless. I have no idea how Lapua gets their areo coefficients, but the 6DOF program isn't terribly useful in the form they provide. It seems like more of a marketing gimmick than an actually useful piece of software.

 

[DocUSMCRetired]

Alas, an Internet search for spin box is heavily contaminated by the computer user interface element of the same name.
Can you point us to a good source describing how a spin box works ? Thanks.

If you have not read it, we have an entire section of lab testing published in "Modern Advancements in Long Range Shooting Vol 1". Where Part 1 is "How Stability Affects Bullet Flight". Chapter 4 of that section specifically is "Spin Rate Decay". https://store.appliedballisticsllc.com/Modern_Advancements_in_Long_Range_Shooting_p/5004.htm. You will find a photo of a spin box on page 60 of this book. You will also find a chapter on bullet stability in our main book. Chapter 10 of Applied Ballistics for Long Range Shooting.

 

[Longrange57]

If you have not read it, we have an entire section of lab testing published in "Modern Advancements in Long Range Shooting Vol 1". Where Part 1 is "How Stability Affects Bullet Flight". Chapter 4 of that section specifically is "Spin Rate Decay". https://store.appliedballisticsllc.com/Modern_Advancements_in_Long_Range_Shooting_p/5004.htm. You will find a photo of a spin box on page 60 of this book. You will also find a chapter on bullet stability in our main book. Chapter 10 of Applied Ballistics for Long Range Shooting.

Page 169
Bryan Litz, "It's extremely difficult to calculate how much spin rate is required to overcome a dynamic instability caused by transonic flight. A few of the reasons are: 1. You can't accurately predict the amount of dynamic instability the bullet will have during transonic flight. 2. You can't accurately predict how much the spin rate of the bullet will have decayed by the time it reaches transonic speed."

 

[DocUSMCRetired]

Page 169
Bryan Litz, "It's extremely difficult to calculate how much spin rate is required to overcome a dynamic instability caused by transonic flight. A few of the reasons are: 1. You can't accurately predict the amount of dynamic instability the bullet will have during transonic flight. 2. You can't accurately predict how much the spin rate of the bullet will have decayed by the time it reaches transonic speed."

That's why we just measure it.

 

[JPeelen]

If you have not read it, we have an entire section of lab testing published in "Modern Advancements in Long Range Shooting Vol 1". ... ...

Thank you for the response. I have the book, but had forgotten that the spin box is described in it.

 

[Ned Ludd]

That's why we just measure it.

Although I'm sure there is quite a bit of variability between various bullets/loads, from actually measuring spin rates, are there estimates for the average decrease in spin per distance versus the decrease in velocity? In other words, is there some rough estimate of the ratio at which spin decreases versus linear velocity? My understanding has alway been that the spin rate decreases much more slowly than linear velocity, and thus bullets tend to have a greater Sg as they travel farther, but I've never seen any numerical values ascribed to the spin rate decay.

 

[JPeelen]

There is 5.56 mm M855 data in a report by SILTON and HOWELL (ARL-TR-5182; dowloadable as ada530985). According to table 9 and assuming normal velocities, after 300 m velocity drops to 68 percent while bullet spin is still at 86 percent of the muzzle value. So the bullet at 300 m does make one turn in about 141 mm of travel, compared to 178 mm at the muzzle.

Swiss ballistician Beat Kneubuehl for 7.62 NATO reported 74 percent remaining velocity compared to 96 percent remaining spin at 300 m. But he does not indicate how he arrived at this result.

 

[Longrange57]

Although I'm sure there is quite a bit of variability between various bullets/loads, from actually measuring spin rates, are there estimates for the average decrease in spin per distance versus the decrease in velocity? In other words, is there some rough estimate of the ratio at which spin decreases versus linear velocity? My understanding has alway been that the spin rate decreases much more slowly than linear velocity, and thus bullets tend to have a greater Sg as they travel farther, but I've never seen any numerical values ascribed to the spin rate decay.[/QUOTE
My observation as well. Since Sg is going up, I guess it's not important to have the info to some and I can see why. Big Stuff section...new build for 1500-1760 I posted what many forums have referred to as Kolbey's formula which is not correct. I contacted Kolbey and he said that it's not his formula. My bad! Wherever the formula came from, someone put some thought into it and had to have some data to build around. This subject gets brought up a lot but like you said, no numerical values. Some feel that there are things going on when approaching subsonic depending on the combination, that we just don't know, yet spin usually enters the conversation.

 

[Ned Ludd]

Yes - I would be happy even just to know something like, "Bullet rotational speed will decrease at approximately one tenth the rate at which its linear velocity will decay"...or something similar to that.

 

[Longrange57]

Yes - I would be happy even just to know something like, "Bullet rotational speed will decrease at approximately one tenth the rate at which its linear velocity will decay"...or something similar to that.

Yes sir, agree