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Why doesn't accuracy always hold up?

 
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  #1  
Old 10-27-2003, 09:29 AM
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Why doesn\'t accuracy always hold up?

Gentlemen,
I've always believed -- as I'm certain many of you have -- that if a given handload in a given rifle yields sub-0.5 or even sub-0.25 MOA accuracy at 100-200 yards, then one can extrapolate this accuracy to as far as the rifle/handload combination will reach, withstanding control of environmental/ meteorlogical variables, of course. But recently, I found two handloads using projectiles of different weight only (same manufacturer, same design, and same construction) that yield approximately the same level of accuracy at 100-200 yards, but which perform very differently at great distance (e.g. 1000 yards). Having just walked into the ranchhouse of a friend's spread here in east Texas where we drove last night to do some 1000 yard shooting with our .300 WSMs, I write you all this to tell you that a Lapua 167-grain Scenar load held up much better than the a Lapua 185-grain Scenar load. And this is the third time now that I've seen this in this rifle and with these same loads.

Why does this happen?!?!?

Dave
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  #2  
Old 10-27-2003, 09:39 AM
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Re: Why doesn\'t accuracy always hold up?

Here are some of the reasons that the smaller bullet is doing a bit better at 1000 yards but, probably would not, if the range were further.
keep in mind some bullets stabilize at different yardages and that could be happening here.

1. The BC is close in both of these bullets.

2. You are getting more velocity from the 167 gr and that may make the difference as per holding up a bit better.

3. Try a comparable weight Sierra MK, JLK or Berger in that rifle once. [img]images/icons/smile.gif[/img]

later
DC
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Old 10-27-2003, 01:30 PM
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Re: Why doesn\'t accuracy always hold up?

DC,

The lighter bullet, traveling at a higher velocity, would have a much higher rotational spin rate when exiting the muzzle. Could this be a factor in its ability to hold true to the initial moa noted at 200-300 yards?

[img]images/icons/confused.gif[/img]

VH
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Old 10-27-2003, 01:44 PM
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Re: Why doesn\'t accuracy always hold up?

VH

Hello
That goes back to stabilization of the bullet.

The two bullets are being fired from the same twist rifle so the rotaional spin is causing a slight stabilization point but, it shouldn't be to any major extent, especially at only 200 or 300 yards.

If you were using a different twist then I would say that the rotaional spin and stabilazaton of the bullet would be much different and noticable at a bit shorter range..

I don't think that is his problem here.

I would try a comparable bullet with a high BC and even shoot further then 1000 yards to see the difference.

Later
DC [img]images/icons/smile.gif[/img]

[ 10-27-2003: Message edited by: Darryl Cassel ]
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Old 10-27-2003, 08:42 PM
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Re: Why doesn\'t accuracy always hold up?

Gentlemen,
Hmmm. So projectiles of different calibers and masses spin at different rates, independent of the barrel's twist? And as for the radical differences in accuracy, even when we begin from very much the same point, this seems to suggest that some bullets actually cork-screw, and not just spin, at different distances from the point they are fired?

Dave
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Old 10-27-2003, 10:02 PM
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Re: Why doesn\'t accuracy always hold up?

The added "velocity" of the lighter projective will change (increase) the spin rate causing an impact change at the target.
Different weight bullets can also stabilize or destabilize at different yardages.

Later
DC
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  #7  
Old 10-28-2003, 12:45 AM
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Re: Why doesn\'t accuracy always hold up?

Dave,

It just may be your 12 twist isn't stablizing the bullet well any longer. In marginally stabilized bullets this will become an issue as it approaches transonic.

I'm fixin to try the 240 SMK in my 11 twist, ya think I'll nail the chrono as they go sideways? [img]images/icons/grin.gif[/img]
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