Bullet Stabilization Questions

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by *WyoWhisper*, Jan 19, 2004.

1. *WyoWhisper*Guest

OK.. too many times I have heard that even though a rifle shoots one hole at 100 yards doesn't mean it will shoot good groups at longer ranges.

Given that you are shooting a quality bullet ( SMK ) and that all conditions are constant. Conditions will be dead calm, 59 degrees, no mirage. The other given is that your twiat rate is sufficient to stabilize the bullet you are shooting.

Someone please explain to me how if the rifle shoots into one hole at 100 why it might not shoot that well down range? If it is holding .5 MOA at 100 what would cause it to hold 2 MOA at 500? taking out all human variables.

some of this just doesn't compute in my head.

2. Varmint HunterWell-Known Member

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Although I can not explain this phenomenon, I have witnessed it several times. My guess is that a shooters ability to see and hold on target comes into play at extended ranges.
Here is a question that is even more difficult to explain. How can a 1moa (@100yds)rifle shoot .5moa @ 500yds?
If I didn't see it, I wouldn't believe it, but it happens.

VH

3. Darryl CasselWell-Known Member

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Ric

One reason is;
There are "always" conditions downrange that you as the shooter may "never" see. Even though you may "think" it's calm, theres something going on that many times you can't explain, see or feel.

Another is;
Bullet "velocity change" (especially out to 1000 yards) downrange can effect accuracy in some bullets. Lets say you start out at 3000 FPS and the group at 100 yds is nice and tight "with that load and velocity." By the time it gets to 1000 yards, it's only going 2000 FPS or less. Ask yourself, what kind of a group would I have shot at 100 yards with "THAT" slower velocity? Would it have been even 1 MOA? That 1 MOA at 1000 yards is 10".

I have seen some rifles that shoot terrible at 100 yards but, did well at 1000. On the other hand, have seen just the reverse.

Just because a group at 100 yards is super tight, it does not mean it will "always" shoot well at 1000 yards. Most times it does but, there are exceptions to the rule. Sometimes it just can't be explained.

Later
DC

4. GuestGuest

Varmint hunter is right , My 300 Dakota shoots just under 1moa at 100yds but will shoot 1/2moa at 625yds. Some think that it just takes a little longer for the bullet to "settle down" or "go to sleep"

Ric as for your question , my way of though would say that the group size should double as the range doubles under perfect conditions , but with focus being a big factor that could cause some change.
but for your groups not to change , you would have to shoot the exact same bullet out of the exact same piece of brass , then your powder and primer would be flawed as their is now way to keep the consistancy exact. Any change wether it be a grain here or thier is going to cause the bullets NOT to follow the exact same path which is what it would take to have a one hole group all the way out.
Its like I have seen guy that shoot BR , do the whole case prep with rubber gloves and hold tolerances to .001" and , weight each one of their bullets to within 1/10 of a grain , use a guage to seat their primers , do everything to ensure that each round is the same as the last , and then what realy got me , the guy uses a powder thrower to get his loads , the cheap *** rig he uses varies the loads as much as a half a grain !!
Somethings arn't ment to be understood I guess.

5. MAXWell-Known Member

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What Darryl said. And gnats. I always blame gnats. Or, what is called dispersion. The minor effects of minute variables become more pronounced as range increases because a) there are more of them(zephyrs, gnats), and b) the bullet is slowing down and the variables have more time to affect them. Other issues mentioned above are at play as well.

As to groups getting smaller as mentioned by a couple of folks, the only things that I think would contribute to that are parallax in the scope and nulling of the fast cycle precession that all bullets carry out of the muzzle. Apparently this occurs within the first 200 yards or so, depending on GS and some other issues that are escaping my memory at the moment.

Anyway, the short form is gnats. I always blame it on gnats. Most folks seem to understand that for some reason or another.

6. jb1000brWell-Known Member

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Ric--i just had a though. Bullet jacket concentricity could also affect the bullets deflection from that one-hole path that it has at 100yards--making the group get inexplicably (sp)larger at longer range.

JB

7. BrentWell-Known Member

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Ric,
An ES of 30 fps in the 308 win with the 175gr SMK at 2600 and 2630 will add over 1 MOA to the group size at 1000 yards, where at 100 yards its effect is only .1 MOA.

At 3000 and 3030 fps it has zero effect at 100 yds, but adds .75 MOA dispersion at 1000 yards.

The faster the bullet is going, or the flatter its trajectory is, the bullets axis following the tangent of its trajectory has less of an effect, that and the range it travels.

It would be interesting to fire groups at nearly straight up or straight down angles to quantify the dispersion effects that the trajectory arc itself introduces. With differnt GS factors also.

As Darryl said, a lot of it comes from minor air disruptions in various directions and intensities that all stack up differently over the entire range to effect each shot quite differently that connot all be seen or accounted for perfectly. I'm not talking the large and noticable wind conditions across the course that are in many ways easily determined, although if not seen and corrected for they will play hell with your groups even more. Of course, even percieved dead calm conditions still may have some air movement across the course, though the effect may be small. They might not be either.

8. Dave KingWell-Known Member

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Ric

This is for external ballistics as I understand it and I see no mention of variation caused by non-homogeneous projectile density (jacket thickness and such) and the forces in place when that type projectile exits the barrel. (A physical center that does not match the density center.)

[ 01-19-2004: Message edited by: Dave King ]

9. MikecrWell-Known Member

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100yds isn't going to show the effects of your ES. (Things like POI shifts due to the barrel coming in & out of it's sweet spot)
I think this is why short range BR shooters can get away with thrown charges. It isn't a measurable effect @100.

I'm in the described above situation. Where I consistently shoot better MOA at longer ranges than short. Can't figure it out.

10. Michael EicheleWell-Known Member

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The really funny thing about the tests I had done stated in the post above all had a very small amount of ES. I get alot of ES out of my best load that I now use and it still shoots .25 MOA to 800 yards. Go figure.

Also those tests were done on multiple days, somtimes weeks apart. Mostly at -10 F when there is absolutly no air movment where I shoot.

No, I cant figure that out either.

11. Varmint HunterWell-Known Member

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Its probably got something to do with the frozen bullets that you're shooting.

VH

12. a.JRWell-Known Member

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WyoW,1 dominant reason to explain the "shoots great at 100 and not at 600 to 1000" is that the bullets are not the same BC as they are pulled from the box....A lot of shooters that strive for better accuracy would spend lots of time trying to get ES's into the single digit figures and then select an unquailified bullet from the box and shoot away...I can assure you that it's easy to get a .2 difference in the BC of a Match style projectile ...If you take a bullet say of a real world BC of .600 and then another that looks the same with a BC of .580 and to this a distinct possibility of a 25 fps spread ,if you shoot it enough,check out how bad this looks on a Ballistic program if you stack the high to the high and the low to the low..This is part of the reason that a lot of guys struggle in trying to get the Vertical dispersion to say a sub 1/2 min level at 1000yds...If your happy with the top bullet 10 inchs higher that the bottom one in your groups then disregard this post...JR..Jeff Rogers..ps This is practial information as i do not even have a Ballistics program to tell you the theory style differences,you would have to run this data yourself..

13. dbhostlerWell-Known Member

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Suprised no one has mentioned bullet path vortices from prior shots.

14. Michael EicheleWell-Known Member

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My take on the matter is that it might have a little to do with BC and velocity. Like most here, I dont know or understand why this might happen. Listed below are 2 examples of bullets that should have been MORE than stable, yet what these 2 examples have in common is their low BC.

Example 1:
I had a load for my 308 win that I dont have anymore. It was a 150 nosler BT at 3100 FPS at the muzzle. It would group ragged 1 holes at 100, and 1.5" at 400 yards. At 600 yards it was 1 MOA or 6 inches at 1k they were sideways. Twist was 12. BC is .435 and it would get worse with velocity decay.

Example 2:
My new 308. Had a 168 grain load at 2600. It would group .25" at 100 and .750 at 300, 1.5 at 400 and 6" at 600 at 1000 they were also sideways. The BC from that bullet in my rifle was .450 Twist was 11.25

Both loads were tested in cold dense air. Both had a poor BC and the one with the highest BC had a lower MV.

Now for the kicker. The 168 grain load was increased to 2661 FPS in the same conditions. It shot .3-.5 MOA at 100 yards. It shot .3-.5 MOA at 600 yards. It shot almost perfectly straight holes at 1k (slightly oblonged) and exactly 1 MOA or 10".

Could it maybe that the velcoity to BC ratio had somthing to do with it?

I also have lower velocity loads with very high BC's and are in the 1.5 GSF zone instead of 2.0 or 2.5 and because they have the high BC they retain their super sonic velocity farther even in the same cold air, they also maintain their accuracy to 1k. I think that bullets "know" for lack of a better term where they are in the speed of sound barrier and accuracy may be invloved. May be there is some relationship between each of these.

Many unknown mysteries here and alot of speculation. This is just what I have found in my tests.

Also these are thoughts and opinions and speculation and guesses, I dont KNOW why this occurs.

Any particular loads this is happening with??

[ 01-19-2004: Message edited by: meichele ]