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.
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? [img]images/icons/confused.gif[/img]
If I didn't see it, I wouldn't believe it, but it happens. [img]images/icons/rolleyes.gif[/img]
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.
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.
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 ass rig he uses varies the loads as much as a half a grain !!
Somethings arn't ment to be understood I guess.
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. [img]images/icons/rolleyes.gif[/img]
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.
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. [img]images/icons/smile.gif[/img]