Bullet stability and short distance

redshooter

redshooter

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First please, I'm not trying to trigger some pissing contest. This isn't about your expert, friend, or Rocket Scientist cousin having all the answers. It's right or it's wrong. I see this stated all the time and frankly it makes no sense to me. I'm coming from that as a person educated as an Engineering Technologist. I know a fair bit about physics, but obviously not enough to answer my own question. So to get to it...

I see this stated all the time, "The bullet hasn't had enough time to stabilize". I've been thinking about this on and off for a long time and frankly it makes absolutely no sense to me. How can something start unstable and then gain stability downrange and act in a predictable way? If it's unstable it's unstable, it shouldn't just suddenly become stable and just happen to be pointing the right way every time! It just doesn't seem possible to me!!!!

I just want to understand if it can happen. Can anyone explain this one way or the other.

Like I said above I'm not trying to bring out the Trolls. If you can't explain it from what you know please just read. Believing and knowing are not the same thing afterall.
 
If you take a hacksaw to the crown of your rifle, the uneven gasses behind the bullet base will make the bullet wobble. A 100 yards later it will have settled down (it may loose it completely however), point forward, like a spinning top that starts off wobbly. The bullet's forward velocity decreases faster than the spin velocity, so gyro stability increases with distance.
 
redshooter said:
First please, I'm not trying to trigger some pissing contest. This isn't about your expert, friend, or Rocket Scientist cousin having all the answers. It's right or it's wrong. I see this stated all the time and frankly it makes no sense to me. I'm coming from that as a person educated as an Engineering Technologist. I know a fair bit about physics, but obviously not enough to answer my own question. So to get to it...

I see this stated all the time, "The bullet hasn't had enough time to stabilize". I've been thinking about this on and off for a long time and frankly it makes absolutely no sense to me. How can something start unstable and then gain stability downrange and act in a predictable way? If it's unstable it's unstable, it shouldn't just suddenly become stable and just happen to be pointing the right way every time! It just doesn't seem possible to me!!!!

I just want to understand if it can happen. Can anyone explain this one way or the other.

Like I said above I'm not trying to bring out the Trolls. If you can't explain it from what you know please just read. Believing and knowing are not the same thing afterall.
Click to expand...

For a given bullet there is an ideal relationship between spin rate and forward velocity. Velocity bleeds off faster than spin rate slows. Hence some bullets will achieve their ideal spin/forward velocity ratio farther down range

This ratio affects Yaw, which affects how closely the each bullet follows it theoretical flight path... see below for a more sophisticated explanation..

http://www.appliedballisticsllc.com/epswerve.html

You could also buy his book and learn alot more
 
Gyroscopic stability factor (Sg)
------------------------------

Sg = Rigidity of the spinning mass (depends on angular velocity)
------------------------------
Overturning aerodynamic torque (depends forward velocity)

Both rigidity of the spinning mass and overturning aerodynamic torque depend on initial velocity but forward velocity declines faster than rotational velocity and this is why Sg increases with time.

Alex
 
roberti11 said:
For a given bullet there is an ideal relationship between spin rate and forward velocity. Velocity bleeds off faster than spin rate slows. Hence some bullets will achieve their ideal spin/forward velocity ratio farther down range

This ratio affects Yaw, which affects how closely the each bullet follows it theoretical flight path... see below for a more sophisticated explanation..

http://www.appliedballisticsllc.com/epswerve.html

You could also buy his book and learn alot more
Click to expand...

So the phenomena can be proven but can't account for wild differnces in groups over various ranges. It applies to a group not growing over distance as it should. For example if it's .75 moa at 100 m it should be .75 moa at 300 m but in fact it's .65 or .70 moa if I've understood it. Not, I've got a group at 100 m that's 1.5 moa and if I shoot the same load at 300 m and it's going to be .75 moa

;) I have the book, just haven't got that far into it.
 
redshooter said:
So the phenomena can be proven but can't account for wild differnces in groups over various ranges. It applies to a group not growing over distance as it should. For example if it's .75 moa at 100 m it should be .75 moa at 300 m but in fact it's .65 or .70 moa if I've understood it. Not, I've got a group at 100 m that's 1.5 moa and if I shoot the same load at 300 m and it's going to be .75 moa

;) I have the book, just haven't got that far into it.
Click to expand...

Keep in mind this was just one example, the degree to which this happens varies according to bullet design/Twist/velocity, I am not sure I have ever seen as much of a difference as you have outlined, but I cannot say that it does not occur
 
Could internet physics be like internet groups...a bit tough to follow the mathematical logic. Until we get "smart bullets" that steer themselves back on course, my skepticism will remain regarding claimed group size reductions over distance. Many shooters fail to admit that the wind can "blow 'em in" as quick as it can "blow 'em out". So, a bullet with an average velocity of 2500fps would make a noticible course correction during the 0.12 second that it takes to travel between 100 -200 yds...until someone can explain how it overcomes that pesky concept of momentum...I'll just remain a skeptic.
 
roberti11 said:
Keep in mind this was just one example, the degree to which this happens varies according to bullet design/Twist/velocity, I am not sure I have ever seen as much of a difference as you have outlined, but I cannot say that it does not occur
Click to expand...

:D What I got from your post was no more than the caliber of the bullet. Just increased the scale for clarity...and the caffeine level is too low for math in my head :D.
 
redshooter said:
So the phenomena can be proven but can't account for wild differnces in groups over various ranges. It applies to a group not growing over distance as it should. For example if it's .75 moa at 100 m it should be .75 moa at 300 m but in fact it's .65 or .70 moa if I've understood it. Not, I've got a group at 100 m that's 1.5 moa and if I shoot the same load at 300 m and it's going to be .75 moa

;) I have the book, just haven't got that far into it.
Click to expand...

I often think that this is caused, in many cases, by parallax error up close...
 
maynard said:
So I guess my next question would be, why do short range benchresters use flat base bullets?
Click to expand...

A variety of reasons really. The big reason is that good quality boat tails aren't easy to come by. A FB bullet is a much easier bullet to make close to perfect than a boat tail. Custom bullet makers hate making boat tails, I used to do it, I hated it. The boat tail punch (not rebated boat tails) is easily damaged, leading to frequent replacement. Also the bullet jackets ship to the maker as copper cups basically. You have to reform the base of the cup to make a boat tail. More chance for imperfections or imbalance to show up in the base of the bullet. A flatbase retains the same original shape as the copper cup.

At our distances the improvement in BC doesn't really add much to the equation. Finding a steady supply of near perfect bullets is more important.

No coincidence the latest bullet from Berger to take on the custom bullet makers in BR, is a flat base. Much easier to produce near perfect bullets.
 
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