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Old November 23, 2007, 08:26 PM   #1
cdoc42
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Join Date: March 13, 2005
Location: Pennsylvania
Posts: 403
Why is it....

....that one .270 (or any other caliber) shoots a Hornady 130gr SST under 1 MOA while another .270 with the same twist rate won't shoot as tight a group until you use Hornady 150gr spitzers?

We have all come to realize that all rifles "prefer" certain loads, and therein lies the beauty of handloading.

But can we examine the factors that are responsible for these differences, if no no other reason than to limit the amount of experimentation one must do to reach that "golden moment" when the shots group under one inch?

I'll start:

1) Differences in the size of the throat
One rifle might have a throat so long that the same bullet loaded at the same depth has as much as a half-inch jump to engage the rifling. But considering most are much less than that, if you change the seating depth of bullets so that the same bullet in two rifles of the same caliber are equi-distant from the rifling, the differences in seating depth change the pressure characteristics of the load, hence the two rifles once again shoot differently.
Can you overcome this by chronographing the loads so they both produce the same velocity? The only change then would be in the amount of powder needed (assuming one doesn't become an overload).

2) Differences in bullet design
130gr bullets in .270 (or any caliber) from Nosler differ from those made by Hornady, Sierra, Speer, etc. The position of the ogive probably has the most influence, but some might argue that the shape - or sleekness - may be a factor. That would tend to make me believe the accuracy is more dependent upon the aerodynamics after the bullet leaves the rifle, meaning your particular rifle has little to do with it. But I DID experience this: My .270 Remington M700 shoots BOTH the Hornady 150gr round-nose and the 150gr spire under one inch with the same primer and load of powder. They are both seated to the same distance from the rifling (leade).

Any other thoughts on this?
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Old November 26, 2007, 11:53 AM   #2
TrimixDiver
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Join Date: March 15, 2007
Location: Central Kentucky
Posts: 12
Several things to consider:

Chamber dimensions
Head space
Throat Length
Land/Groove Dimensions (slug the barrel)
Twist Rate (in your case they are assumed identical)
How smooth are the lands/grooves (bore scope needed)
Barrel Harmonics (Length, Diameter, Free Floated, Etc)
Bullet Shape (varies by manufacturer)
Bullet Jacket Construction ( again varies by maker - Thicker/Thinner Jackets)
Bullet Base (Flat vs. Boat-tail)

With regular production rifles, the tolerances for manufacturing can account for why one model might be a tack driver and another of the same model might be an also ran with the same ammo.
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Old November 26, 2007, 12:32 PM   #3
Mike Irwin
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Each rifle is an individual, and likes different things.
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