Borderline Stabilization... at long range

[82nd airborne]

Spin drift is also a factor at extended ranges, im guessing to to the same thing?

[Eutectic]

Hi Sagacious,

I have taken the liberty to pull a few of your comments from your post. I have responded with my thoughts to them in blue. Please respond with your thoughts especially where I am wrong.

"I suspect that you mean gyroscopic stability along the course of the trajectory."

Yes... Stability along the centerline of the projectile axis.

"No, the second rpm figure is not correct for 530yds."

Yes! That's the point I was after! IT IS NOT THE CORRECT RPM AT 530YDS.

"Calculating rpm based on at-distance velocity is not accurate."

True.

"The rpm at muzzle is not "locked in", it's simply the rate with which it begins."

By 'locked in' I mean the values induced by gas pressure and rifling twist are over at the moment our projectile leaves the lands and grooves... The values of forward speed and rotational speed are 'locked in' and measureable... This does not mean they are 'locked in' after our disconnect! If fact, rotation and forward travel are not 'locked in' the same again. In the atmosphere each dynamic (rotational and forward travel) has its own set of variables and this is where confusion sets in.

"Rpm and forward velocity are two properties that do not diminish in sync, as easch is subject to distinctly different forces."

This sentence seems to contradict itself... Maybe 'diminish' is confusing me. Could you please explain again?

"Note that there seems to be a slight confusion of terms: Rate-of-twist (rot), rate-of-spin (ros), and rpm are not synonymous. Unlike rpm, rot does indeed start as an invariant factor. Rpm will decrease as forward velocity slows, and ros will decrease as it's angular momentum slows. Rot does not change, because rot is simply the rifling pitch rate, and that can't change unless the gun is rebarrelled. Rot = ros at muzzle, but not at distance. This fact is at the very heart of your question."

Your (rot) is my 'locked in' and is only relevent while our boolit is still engaged in the rifling.. Your (ros) muddies the waters with confusion and is probably not needed.... Downrange Boolit RPM (rotation over time) coorelated with Boolit SPEED (distance over time) will allow computation of our HELIX LENGTH where ONE full 360 degree rotation of our boolit has taken place. THIS IS the very heart of the question...

Quote= (OK... A tougher question.... At 530 yards does our bullet still travel 14" to make one full (360 degree) revolution?)
"No, it travels slightly further than 14" to complete a full revolution, because the ros at distance has slowed from the rot at muzzle."

I disagree with you here. Please explain how the 14" HELIX (again I use the term 'HELIX' in our case for the length of ONE full 360 degree rotation of our boolit) holds constant over 530 yards as two different sets of variables are at play, one for rotation and one for forward speed?

Quote=(Let's say we lose 20% of our RPM over 530 yards or more easily looked at we have 80% of our original RPM that we had at the muzzle. That figure is 82,286 RPM. I think it could be higher... we are only talking 1.175 second of flight time here.
So if you allow me the 82,286 RPM figure and we calculate this against our 530 yard retaining velocity of 1000fps to get a theoretical 'twist' for these numbers the twist would then be the equivalent of 1 turn in 8 7/8" for our boolit!!!!!

"If the rpm slowed by 20%, then the bullet would have to travel 20% further to complete a revolution, not 20% shorter."

I don't think so. What is missing here?

"Also, taking for granted your rpm figure, still, the ros calculation at 530yds is not correct. Your math shows the bullet revolving faster than 1:14" at distance. Where did it's ros get the energy to increase from it's initial rot? Of course, the ros cannot speed up unless energy is being added. Rot is always >or= ros."

I believe we are looking at less rotational energy lost, not energy added for the RPM figure. I think we agree at the bullet's departure from the rifling that for our example the RPM is 102,857. I used a loss of 20% of rotational speed over 530 yards. I believe the loss is even less.... so does Larry Gibson.... Regardless of our opinions, 80% of the RPM at muzzle is the 82,286 RPM figure. We cannot forget velocity (our forward speed) at 530 yards is now only 1000fps.... This could have happened?

I see this happening at 530 yards for our example.....

82,286 RPM divided by 60 = 1371.43 revolutions per second. (RPS)

1 divided by1371.43 RPS = .00073 second (the elapsed time for one revolution)

1000 feet per second remaining velocity X 12 = 12,000 inches per second.

12,000 inches X .00073 second = 8.76" travel for one bullet revolution or the length of our 'HELIX' as I am using the word in our discussion.

Again, please explain how your 'ros' remains a constant while our other dynamics are changing continually along our boolit's flight.....


"Just hoping to clarify, and respond to the questions given. Hope this discussion stays friendly."

I do as well... I look forward to your thoughts!

Eutectic

[felix]

We should never assume the boolit is actually doing the twist upon muzzle exit according to the calculated spec. Here we enter the 44man's arena of his saying the faster twists in his revolters are proving to be "mo'betta". ... felix

[Eutectic]

felix,

I understand your point of some slip and slide in the high pressure .44 loads especially in short length barrels.... But don't you think our mild rifle load example in a 24" barrel with a fairly slow twist would be fairly close?

Eutectic

[sagacious]

Hi Sagacious,
Yes... Stability along the centerline of the projectile axis.

That is probably not what you mean. The bullet can be stable along it's axis, but unless the gyroscopic axis roughly follows the trajectory, the bullet will not be stable in flight. The bullet can spin just fine along it's own axis, but unless the two axes coincide for the nduration of flight, the bullet may become dynamically unstable. Stability along the bullet axis does not equal stability in flight over time.

"Rpm and forward velocity are two properties that do not diminish in sync, as each is subject to distinctly different forces."

This sentence seems to contradict itself... Maybe 'diminish' is confusing me. Could you please explain again?

Diminish = decrease. Rpm and fv do not decrease in synchrony; thus calculation of rpm cannot be determined by velocity at distance. For rpm to be calculated precisely, one has to take into account the loss of spin over time. Standard calculations of rpm give only the accurate rpm at muzzle.

Downrange Boolit RPM (rotation over time) coorelated with Boolit SPEED (distance over time) will allow computation of our HELIX LENGTH.

This is in error, because the ros at 1:14 lengthens over time, and is totally independent of downrange velocity. For example, a bullet fired perfectly vertically will reach 0fps at the apogee of it's trajectory, but will still be spinning very fast. Ros and velocity are in no way coupled.

I disagree with you here. Please explain how the 14" HELIX (again I use the term 'HELIX' in our case for the length of ONE full 360 degree rotation of our boolit) holds constant over 530 yards as two different sets of variables are at play, one for rotation and one for forward speed?

The term 'helix' is not used correctly for this discussion. The bullet in flight usually describes a helix with it's point or cg relative to the trajectory. Ros is what you're describing, as the helix and ros or rpm are unrelated. It may be better to use a term other than helix.

Rotation is also correct, but revolution is not correct for this discussion. For example, the Earth rotates about it's axis, but it revolves around the Sun. The Earth does not revolve about it's axis, nor does a bullet. Rotation and revolution are not synonymous. Just a point to help clarify.

The 1:14 ros does not hold constant; it must lengthen over time as the angular momentum of the bullet decreases. Put another way, the distance over which the bullet makes one full rotation increases over time as drag slows it's rate of spin.

Ros may be the best term, as we're describing the rate at which the bullet spins (such as 1:14")-- same as we use twist rate (rot) for the rate at which the rifling turns (such as 1:14").

1000 feet per second remaining velocity X 12 = 12,000 inches per second.

12,000 inches X .00073 second = 8.76" travel for one bullet revolution or the length of our 'HELIX' as I am using the word in our discussion.

Again, please explain how your 'ros' remains a constant while our other dynamics are changing continually along our boolit's flight.....

As I noted above, ros cannot remain constant. It lengthens over time.

I checked your math very carefully. The problem is that you're still using downrange velocity to calulate the rate at which the bullet spins (1:8.76"). This will not work, as the rate of spin and rate of fv decrease happen at different rates, so one cannot use the downrange velocity to calculate the distance the bullet travels to spin once (ros).

I suspect there may a measure of miscommunication by either one or both of us. If I'm misunderstanding your questions, just let me know.

A pleasure discussing with you, regards and good shooting!

[rvpilot76]

Here's a thought:

If you have an unbalance tire and travel at say, 45 MPH, you feel the shake in your steering wheel. Now take that same unbalanced tire and travel at 70 MPH. In my experience it's not as noticeable. Think of the shake in terms of frequency. At high speeds, the frequency is so high that it is almost imperceptible. At low speeds, however, it is. I may be off here, but think of the frequency cycle as a direct correlation to group size. The slower the round, the lower the frequency, the greater the spread of the frequency between the peaks and valleys of the frequency, the greater the group size. This explains, in some way, the loss of accuracy (or increase of frequency "spread") at longer ranges. Not a fix all, but I hope I have increased your understanding of the subject matter involved.

Kevin

[44man]

When we shot IHMSA, we watched many boolits and bullets in flight. We seen some mighty strange things.
I do not know the twist rate of the S&W 29 but the 240 gr bullets would rotate around the flight path like a corkscrew with the powder charge that was the most accurate. It seemed to settle some at long range but it gets very hard to track the bullet with a spotting scope.
This did not affect accuracy and I shot many, many 1/2" groups at 50 meters but it would change POI at other ranges, not enough to worry about.
By going to a 250 gr bullet or boolit, this rotation was gone. The Rugers did not have this rotation at all and boolits flew a straight path if you discount drop.
The BFR's have not been watched but since heavy boolits are used, when sighted at 50, they stay on line to as far as I have shot them which is 500 meters, a lot of drop but accuracy is still there. I do know the guns do not like real light boolits for the calibers while the Freedoms and Rugers prefer a lighter boolit because of the slower twists or more powder is needed for a boolit weight increase. Not drastic, maybe 1/2 gr more powder then the BFR.
Then there was the TC 30-30 with a cast boolit. It was accurate but when watching the boolit go downrange, it would go sideways like it was going to hit the next ram over, come back on line and hit the one aimed at. It caused a lot of laughter and we took turns watching it. The boolit was not wobbling or rotating, it just veered off. At other distances, it hit a different POI on each target, only windage.
Now my .220 Swift with 60 gr bullets. (Pre 64 model 70.) I never shot a group at 100 under 1", mostly larger. I sighted at 350 yards only and head shot chucks beyond 600 yards.
One day while sighting I shot a five shot 1/4" group at 350 yards. A fluke---maybe! But I learned the bullet did not "go to sleep" until out there. The over spun bullet was an advantage at long range.

[44man]

Now I have to look at my Browning BPCR with a 1 in 18" twist. I have every weight boolit you can think of and have shot some 200 meter groups of 1" from the bench but I can not hit the 500 meter rams. In years I have not discovered the right boolit length or velocity because velocity is limited with BP. Sometimes I did good but any change at all would be a total loss.
It has to be the hardest sport ever and it seems only certain guns work and the same guys win all the time. Do they have a faster twist? They will not say.
My BFR 45-70 with a 1 in 14" twist will out shoot the rifle to 500 meters BY FAR. If it was possible to adjust the sights for each range and I was allowed to use it, I bet I could shoot a 40. To hit steel I have to aim at a tree limb about 26' over the ram. Each ram does not have THAT tree limb!
I hate math, I hate Greenhill, I hate predictions! I go by what I see and shoot only.
Now the military has a rifle with a computer that figures in everything before pulling the trigger so a one shot kill can be made at extreme ranges. Makes a guy feel real stupid.

[felix]

Eutectic, I hear ya' loud and clear, and your reasoning seems right on. The fly in the ointment comes when we take the many DIFFERENT batches of boolits and they can be seperated ONLY by the target results. This means each boolit within a group was as identical as is possible by home means. Weight, hardness, sizing, lube, casting condition, and you can name the remaining. The only thing left was the rotational results of each boolit BEFORE and AFTER being shot.

I never get tired of thinking about the Iowa class of guns. I remember the Navy(?) asking for sign-up interviews at MU of its graduating seniors, 1963 in math/engineering. The job was working on "Fire Control" applications. At that time I was surely not interested in putting out fires on Navy ships, so what a mistake on my part for not at least asking the question of someone in the know about just what are they talking about. Now, of course, I know that would have been one interesting job for me. Just think, a moving bench with a moving target up to 26 miles away, and hitting that football sized target on every shot (a turrent's worth?). ... felix

[44man]

Eutectic, I hear ya' loud and clear, and your reasoning seems right on. The fly in the ointment comes when we take the many DIFFERENT batches of boolits and they can be seperated ONLY by the target results. This means each boolit within a group was as identical as is possible by home means. Weight, hardness, sizing, lube, casting condition, and you can name the remaining. The only thing left was the rotational results of each boolit BEFORE and AFTER being shot.

I never get tired of thinking about the Iowa class of guns. I remember the Navy(?) asking for sign-up interviews at MU of its graduating seniors, 1963 in math/engineering. The job was working on "Fire Control" applications. At that time I was surely not interested in putting out fires on Navy ships, so what a mistake on my part for not at least asking the question of someone in the know about just what are they talking about. Now, of course, I know that would have been one interesting job for me. Just think, a moving bench with a moving target up to 26 miles away, and hitting that football sized target on every shot (a turrent's worth?). ... felix

Amazing what the military has done. How great to know all of it. But at today's dollar value, we are talking BILLIONS of tax payer dollars when we complain about over $20 for a can of powder.
Wouldn't you love to run full speed through the woods and hit a deer also running full speed?
No, I don't think so, it is not hunting or personal ability. Might just as well hunt with a grenade. There is just something right about shooting a revolver off hand and dumping a deer to 100 yards instead of putting the gun in a turret with computer aiming.

[felix]

All sports have their intentional limitations, and deer hunting is no less a sport. I kinda' wonder about using scopes, but then we are branching into varmint hunting. Start using benches, we are talking about targeting groups. I have quit group shooting in the last few years, and just use KY windage, scope or no scope. I get the scope into range by shooting once and moving the crosshairs to match the target. ... felix

[Eutectic]

I hate math, I hate Greenhill, I hate predictions! I go by what I see and shoot only.

I love it 44man! ... You have convinced me to drive over to Billings and borrow a gun from an old friend. I don't have a .250 Savage in 14" twist but he does....

I need to answer a few questions for myself... Let's see what gun, caliber, load, boolit, velocity, twist, and 530 yards have to add to this hypothetical example we have kicked around like a football from the keyboard in this thread!

By the way..... What is the rifling 'twist' rate in the 16" guns used in the "IOWA" class of battleships?

Eutectic

[felix]

Don't know about the twist, but it is guaranteed to be microgroove. The lands/grooves are a triangle in shape. Looks like an inch deep or better, when looking up at a fairly straight barrel overhead. ... felix

[44man]

Now look at the tank rounds. Sabot's and smooth bore barrels. Hit another tank while going balls to the wall over rough country.
We need fin stabilized boolits.

[felix]

It would be very interesting for me to see what parameters, both input and output, the computers are using from and including targeting through firing. I know how the programs work, but not the data. I must assume GPS is involved big time nowadays. ... felix

[44man]

It would be very interesting for me to see what parameters, both input and output, the computers are using from and including targeting through firing. I know how the programs work, but not the data. I must assume GPS is involved big time nowadays. ... felix

Not me. I repaired TV's and radios part time for many years. I majored in auto mechanics in school. I worked for an airline for 42 years.
But today I feel like I crawled out of a cave and discovered ---ROCK!

[Larry Gibson]

sagacious

........ The bullet can be stable along it's axis, but unless the gyroscopic axis roughly follows the trajectory, the bullet will not be stable in flight. The bullet can spin just fine along it's own axis, but unless the two axes coincide for the nduration of flight, the bullet may become dynamically unstable. Stability along the bullet axis does not equal stability in flight over time.

Just a bit of further clarification regards to inaccuracy;

The bullet doesn't necessarily become dynamically unstable . If the gyrascopic axis is aligned with but not coaxial with the trajectory the bullet travels in a spiral around the axis of trajectory. The greater the imbalances of the bullet and the greater the centrafugal force (higher RPM) the greater the spiral around the axis of trakectory. This is fundementally why bullets shoot groups and do not shoot into one hole.

Many erroniously think that "stability" means the bullet is pointing forward and any inaccuracy caused by "instability" means the bullet is unstable and tumbling. Such is not the case. The rest of your post is very well put and correct. I

Larry Gibson

[sagacious]

The bullet doesn't necessarily become dynamically unstable . If the gyrascopic axis is aligned with but not coaxial with the trajectory the bullet travels in a spiral around the axis of trajectory.

Larry Gibson

Larry,
You are of course right on. I thougth about that aspect when I replied, but decided not to add precession to the mix. In fact, and as you know, because of the magnus force, the bullet never flies perfectly point forward, but always slightly off. We're probably not at that point in the discussion yet.

Best regards to you and yours.

[c3d4b2]

Thanks to Google you are able to read Dr. Franklin Mann's "The Bullet's Flight"on your computer


http://books.google.com/books?id=QdQ...age&q=&f=false

[NSP64]

My head hurts


I used to shoot RB out of my ML 1-24 twist. I just adjusted powder down untill it started shooting tighter groups.