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Surviving the transonic barrier.

 
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  #8  
Old 01-30-2010, 06:21 PM
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Re: Surviving the transonic barrier.

Quote:
Originally Posted by elkaholic View Post
In addition to things already mentioned, I believe the ratio of the weight FORWARD also helps.........Rich
That has been a long time theory of mine. The problem is that it would be hard to make a bullet with a very high BC and have a nose heavier than the rear. I am also of the opinion that bullets with a super high Sg factor can potentialy have more trouble making it through as any defects or concentricity issues could create a minor amount of imbalance during it's flight which when subjected to the effects of making the transition could aid in the bullet starting to destabilize faster whereas a bullet that is truely asleep may start to wobble a bit less than one that is already down that path.

Understand that the above is ALL theory and I am not saying that this is how it is. I have no evidence to back any of it up.
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  #9  
Old 01-30-2010, 06:27 PM
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Re: Surviving the transonic barrier.

The round ball has a perfect shape and center of gravity
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  #10  
Old 01-30-2010, 09:05 PM
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Re: Surviving the transonic barrier.

Decreasing the twist rate makes the bullet less stable at range, especially as the bullet passes into and through transonic (800-1100 fps) speeds. The reason for this is that unless the bullet has a high twist rate through the transonic speeds it does not have sufficient rotational momentum to stabilize itself through the intense turbulence created at transonic speeds. The instability presented by slower twist rates makes a downrange projectile inherently less accurate and effective. But spinning a bullet too fast also presents some problems.

To read the complete article see link:

http://www.mamut.net/markbrooks/newsdet37.htm
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  #11  
Old 01-30-2010, 09:19 PM
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Re: Surviving the transonic barrier.

Personally I dont totally agree with the thought that slower twists are the culprit. A slower twist doesnt meen instability at range. It is either stable or it is not stable. As Litz pointed out in another post, the Sg improves over time. If it does not leave the muzzle stabilized, it wont stabilize at all. If it leaves stabilized, it will only get better.

Below is a post on SH. It is very interesting to read the above link and then Bryan's post.

"The Greenhill formula was good for the projectiles and mostly subsonic speeds of it's time. In the modern world of pointed, boat tailed bullets fired at 3+ times the speed of sound, the Greenhill formula is simply not very representative, even with the attempts at modern correction factors.

The new standard for practical stability calculation is the method developed by Don Miller. This is the method I recommended for use in Gustavo Ruiez's program: LoadBase2 (mentioned above). Millers stability formula is essentially a curve fit to empirical data (not a direct stability calculation). As such, it has it's limitations, but is quite accurate and very useful for estimating gyroscopic stability. I have written a simple program that calculates stability based on Millers formula that I will be happy to email to anyone interested (free).

The Miller formula is the most practically useful formula for estimating stability, but there are more accurate ways. The McGyro code, developed by Bob McCoy is more accurate, but is still not a direct calculation of bullet stability. It's a more sophisticated empirical estimate that involves more variables, some of which not available or obvious to the average shooter. The JBM twist calculator runs the McGyro stability code, and can be used (for free) at this address:
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One important note on the use of both the Miller and McGyro programs:
It's a very common mistake to think you can calculate the downrange stability with these programs. The programs are good for calculating muzzle stability only. Even though the JBM implementation of McGyro gives stability factors at what appear to be 'downrange' velocities, that's not the case. When the output says: SG=x.xx at 1300 fps, it means if you fired the bullet with a muzzle velocity of 1300 fps, the SG would be x.xx. It's NOT the SG of the bullet fired that's fired at high velocity and slows to 1300 fps.

Historically, Walt Berger has established the twist requirements for Berger bullets and continues to do so. For the few bullets that have been released in my short time with Berger so far, Walt and I have discussed our independent calculations of twist requirements based on different methods and have found them to be in agreement. My methods are Miller and McGyro. I don't know for sure what Walt's prediction method is based on, but it's agreed with my calculations so far. We always round the number down to the faster twist to be on the safe side, which is why many times (especially at high altitude in warm air) people find they can stabilize bullets with slower than the recommended twist.

Beyond the Miller and McGyro prediction codes you would have to use a direct calculation that requires the actual aerodynamic and mass properties of the bullet (the 6-DOF details that muffcook was describing). These are time consuming and difficult to generate, even for someone with the resources and knowledge to do it. Honestly, the prediction methods (Miller and McGyro) are close enough that a more sophisticated calculation is seldom required, especially since we leave a safety margin.

Moving on...
You can't tell anything about stability from rotational speed (RPM) alone. Calculating RPM's as a means to quantify stability is a common mistake. The gyroscopic stability factor (SG which is calculated by the Miller and McGyro programs) is the real measure of bullet stability. RPM's play a part, but is not the whole picture.

Consider the conflicting influences involved in bullet stability. The destabilizing influence exists because the center of pressure (cp)is in front of the bullets center of gravity (cg). Airplanes, rockets, and arrows achieve stability by forcing the center of pressure behind the center of gravity with tail surfaces. Bullets, however, have to live with their cp in front of the cg and achieve stability another way. The stabilizing influence for a bullet is it's spinning mass. The spinning mass makes the bullets axis rigid, and resistant to the destabilizing overturning torque of the cp being in front of the cg.

Deep breath...

Now consider the factors that affect the relative strength of the stabilizing and de-stabilizing influences.
1. Increasing twist rate (while leaving everything else unchanged) will increase the rigidity of the spin axis while leaving the de-stabilizing influences unchanged, so stability is improved.
2. Increasing the length of the bullet will usually increase the distance between the cp and cg which increased the destabilizing torque which has a destabilizing affect. Also, the longer bullet has a less rigid spin axis than a shorter bullet.
3. Increasing the weight of the bullet while leaving everything else the same will increase the rigidity of the spin axis.
4. Now we get to the interesting 'double edged sword': velocity. Increasing the velocity has two effects. First, it will increase the RPM's of the bullet which has a stabilizing effect. Second, it increased the force (aerodynamic drag) that's applied at the cp thus increasing the overturning (destabilizing) torque. The net result is that increased velocity improves stability because the spin axis is strengthened a little more by the extra RPM's than it's weakened by the greater overturning torque. The increase in stability with velocity is far less than a 1:1 correlation with velocity though.

All of the above has been talking about gyroscopic stability, specifically at the muzzle. Gyroscopic stability will generally improve as the bullet flies down range because the forward velocity (a de-stabilizing influence) is eroding faster than the spin rate (a stabilizing influence). If a bullet has gyroscopic stability at the muzzle, it will only grow larger downrange.

It's interesting to note that the gyroscopic stability factor is the ratio of the stabilizing influences to the destabilizing influences. So in theory, this ratio should be greater than 1.0 for the bullet to be gyroscopically stable. In practice, you want SG to be a little greater than 1.0 to allow a margin for error (imperfect calculations, imperfect barrel twist, non-standard atmospheric conditions, etc).

Now to the really interesting question which prompted me to start this thread.

I said earlier that gyroscopic stability only increases with range as the bullet slows down. So why do bullets have stability problems at transonic flight speeds? When a bullet has 'trouble' at transonic speeds, it's not for a lack of gyroscopic stability, but rather dynamic stability which is more complicated than gyroscopic stability. I say it's more complicated because the factors required to calculate it are so very hard to predict, that even if you have a 6 DOF simulation, the parts of the aerodynamic model that are important for predicting dynamic stability can only be determined within ~+/- 20%, and in some cases can be as far off as 100%! Transonic aerodynamics is messy business and is a very difficult challenge for modeling and simulation. When I was 'greener', and had more faith in the modeling tools than I should, I was predicting transonic stability for bullets with confidence. It took a few cases of being proven wrong, but I learned the limitation of the modeling tools for this application. The truth is, you just have to try it before you know if a particular bullet at a particular twist, MV, atmosphere, etc will successfully negotiate the transonic regime.

In summary:
We have Miller and McGyro (both free and available) to predict gyroscopic stability at the muzzle. These are predictions, but are accurate enough for selecting a proper twist for gyroscopic stability. The equations of motion in most ballistics programs are direct, exact calculations for ballistic trajectories which are more accurate then commonly believed. The equations assume the bullet is flying point forward, which it will for a broad range of stability, until it approaches transonic. At that point it's a gamble. Generally, heavier bullets (high BC) are able to negotiate this flight regime better than light (low BC) bullets, but there are no hard and fast rules.

I hope this answers more questions than it raises!

Take care,
-Bryan"
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Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

Last edited by Michael Eichele; 01-30-2010 at 09:29 PM.
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  #12  
Old 01-30-2010, 09:57 PM
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Re: Surviving the transonic barrier.

Lots of good information, well done fella's.

Ok so lets look at two similar bullets.

The .338 calibre 250gn SMK and the 300gn SMK. Both have similar shape and form factor, but one is heavier and longer than the other.

Fired from the same rifle with a 1 : 10 twist. The 300 gn at 2850 fps and the 250 gn at 3100 fps.
Out of the two. which one do you think would better survive the transonic barrier and why?
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Old 01-30-2010, 10:15 PM
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Re: Surviving the transonic barrier.

Quote:
Originally Posted by Topshot View Post
Lots of good information, well done fella's.

Ok so lets look at two similar bullets.

The .338 calibre 250gn SMK and the 300gn SMK. Both have similar shape and form factor, but one is heavier and longer than the other.

Fired from the same rifle with a 1 : 10 twist. The 300 gn at 2850 fps and the 250 gn at 3100 fps.
Out of the two. which one do you think would better survive the transonic barrier and why?
My guess is niether. If high BC had anything to do with surviving it the 300 would have a better chance. If higher than average Sg and shorter bullet had anything to do with it the 250 would. However, based on past experience using a 1.9 stability factor with a shorter than average bullet, they still destabilized at the transonic range. So either it was over spinning them that did it or it was other factors. Regardless, a very high Sg didnt save it. However, it would be very interesting to see if one or the other survived. Best case, they both do. Worst case niether do. Better yet, one or the other. This would give us an indication as to whether it was a higher BC or a higher Sg that did the trick as using your example would show that. The 250 would have a conciderably higher Sg which if it wouldnt make it would show that too high an Sg would cause destabilization. If it survived it and the 300 didnt it would show that a lower Sg regardless of BC isnt the answer where the higher Sg is the answer. If only I had a place to shoot 2000 yards :(
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Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

Last edited by Michael Eichele; 01-30-2010 at 10:24 PM.
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  #14  
Old 01-31-2010, 06:08 PM
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Re: Surviving the transonic barrier.

I shot the Hornady 250gn BTHP Match bullet this morning at 2285 yards. (as per my other post).

The Hornady performed very much better than the Sierra 300gn SMK. The impact zone was much smaller than that of the SMK bullet and quite predictable. There were no bullets that landed way low like occured with the 300gn SMK.

The results from shooting at this distance seem to indicate to me that a shorter bullet, fired at a higher velocity with a bit higher RPM, is better able to survive the trip through the transonic barrier.
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