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Bullet stabilization myth?

 
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  #1  
Old 04-06-2008, 12:20 PM
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Bullet stabilization myth?

I've been lurking around these forums for some time now and have read a lot of amazing, very useful information! I am getting ready to work up a load for my 7mm RM with the berger 168's and I had a question about something I read.

I've read in several places in these forums that one must sight in for at least 300 yards so that the (high bc) bullet will have a chance to stabilize. I read that at 100 yards you can have 1 or higher MOA but that at 300 it could get better - or visa versa.

My question is: How can this be??? Your bullet does not have fletchings on it like arrows. I always thought your bullet is either stabilized right out of the muzzle because of the right twist rate in your barrel (and the quality of your barrel), barrel harmonics (the right node), or it's not. Either the quarterback throws a lame duck or a tight spiral - not a lame duck that suddenly turns into a tight spiral.

How can a bullet that is on a plus moa path at 100 yards suddenly be .5 moa at 300? It can't just change directions from it's established course. I understand sighting in at 300 yards (and 800 plus for that matter) for reasons of validating your trajectory or measuring extreme spreads, etc. But allowing your bullet to stabilize???

Can anyone offer an explanation based on real ballistics or physics?
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  #2  
Old 04-06-2008, 12:36 PM
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That is not accurate. That info is b.s.. You can site in at any range you want. The standard is a 100 yards.
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  #3  
Old 04-06-2008, 01:48 PM
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drenge:

Read this thread for some detailed explanation and information on this topic. It's titled How do you lower S.D.? You may change your mind...

Here's the link:

How do you lower S.D.?
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Old 04-06-2008, 03:24 PM
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Quote:
Originally Posted by linksmechanic View Post
That is not accurate. That info is b.s.. You can site in at any range you want. The standard is a 100 yards.
This is not BS info and it has been well documented that some loads will indeed shoot a higher MOA at 100 yards than they will at longer distances. There are several theories as to why this occurs

Yes you can sight in at any distance that you choose but, sight in distance was not the question, the question was the best distance to accuracy test your long range loads and I definately agree that 100 yards is too close.. I have tested loads that for all intents and purposes shot about the same moa at 100 yards but one bullet shot a lot elower moa at distance, the revers can also be true..
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Old 04-06-2008, 03:25 PM
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phorwath:

I appreciate the post you sent me. I read most of it and agree with a lot of it. However, one member said:

"I have witnessed many times that shooting VLD type bullets at 100yds they still exibit "yaw" they have not quite gone to sleep yet. They might only shoot 1 MOA at 100 yds but after fully stabilized will shoot 1/2 MOA at 500 or 1000yds."

It is comments like the one quoted above that makes me very confused. Even if there is yaw out of the muzzle, isn't it going to be consistent "yaw" if your load is set up properly (ie, proper load density, powder, seating depth, low runout, etc)?

It's hard for me to wrap my mind around a bullet that is prining plus moa at 100 yards and then suddenly .5 moa at 500 or 1000... If you're .5 at 500 doesn't it make sense that the bullet is very well loaded in a properly set up rifle (bedding, etc) and that it will follow that same path as it passes the 100 yard mark even if there is yaw? Meaning I thought that it has to be accurate at 100 if it's going to be accurate at long range?

I could, however, see a bullet that is accurate at 100 but not accurate at all at 500 because of your standard deviation of velocity. If your SD (or extreme spread) is large it will be more noticeable at longer range.

But I asked this question about bullet "stabilization" because I would like to better understand how or why a bullet can be 1 moa plus at 100 and then .5 moa at longer distances.

I still don't get it...
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Old 04-06-2008, 04:08 PM
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I as stated above their are a few different theories as to why this occurs but make no mistake it does occur...
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Old 04-06-2008, 04:34 PM
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There are others out there with much more scientific knowledge then I have on the subject. What I have is practical real world experience and this is what I have learned from shooting very long bullets to high velocity in fast twist barrels. Here are a few things I have learned:

The bullet is not unstable when it leaves the barrel, BUT, what is happening is that as the bullet is in the barrel, it is forced to rotate around the axis of the bore. When the bullet exits the bore, the bullet then will try to spin around its center of gravit. This is seldom exactly the same point as the axial center of the bore. AS such, there is some "wobble" if you will in the bullet.

This happens until the bullet settles down or "goes to sleep" and spins true around its center of gravity. Think of it like a childs toy top. When you first release it it will have some wobble and it will also move around a bit on the ground, then it stabilizes and spins much more even and true. Same thing happens with the bullet.

here are some general things I have found to be true:

1. The longer the bullet, the more range it takes for the bullet to fight off the effects of the rifling and spin true around its own center of gravity.

2. The faster the twist, often the more you will see this.

3. The higher the velocity, often the more you will see this.

If you take a conventional bullet, it also does this but the relatively low weight of the bullet and its relatively short length do not show this as far down range

For example, a 50 BMG will often take 250 to 300 yards for the bullet to completely go to sleep and spin true. My 7mm AM for instance will be very similiar to this shooting a 200 gr ULD RBBT as will the 338s based on the 408 CT case.

In most cases, with conventional chamberings, this range is more like 100 to 150 yards. In most varmint chamberings with low bullet weight, its generally well under 100 fps.

So how do you know?????

Well, the best way is simply to measure groups at various ranges. If the rifle is accurate and the load is a good consistant load, its relatively easy to do. For example, many of my personal heavy rifles will shoot 1/2 moa at 100 yards. At 300 yards, the group size will often be 1/3 moa. At 500 yards, its not uncommon to see 1/4 moa. That this tells me is that once you get past 300 yards or so, the bullets are generally shooting nearly the same or smaller moa group levels out to very long range.

It is not uncommon at all to see this type of rifle shooting very long, very heavy bullets at high velocity out of a fast twist barrel to see a load that shoots 3/4 moa at 100 yards that can shoot 1/2 moa or even less at 500 yards.

Most believe that once a bullet is sent on its path in flight, the groups it forms will be conal in shape. That is generally not the case with a heavy, long bullet.

Now this is really a simplistic way to look at it but it happens time and again. It has nothing to do with stability really, its just the bullet trying to fight off the forces impossed by the rifling and fly true and spin around its own center of gravity.

Again, I am sure many will read this and say the terminology is wrong and I am sure it is but I believe the ideas are sound from what I have seen testing many rifles with this type of bullet.

With conventional weight and velocity bullets, it will be much harder to see if possible at all.

Kirby Allen(50)
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