what causes this?

[PaddyD]

Here's a question I have been wondering about for years now.
What causes entire groups to shift left to right between bullets of the same style, weight and profile, but of different brands? Both loaded in same cases, sized and trimmed the same. Same powder and charge. Same primers. Everything being the same with exception of the brand of projectile, what causes one to be dead in the bullseye and the other to print 2" to the right?

[darkker]

"It's never lupis..."

Things are never the same. Unless: The bearing surface, ogive, concentricity, jacket thickness, jacket alloy, lead allow, concentricity, powder lot, are EXACTLY identical. Then things are different, and the results as well.

[PaddyD]

I guess I should have said very nearly the same. Still, what would cause two 150 gr spitzer bullets to group at 100M like that? Both groups were right at .75" but one in the eye the other 2" to the right.

[stomp442]

Just like darkker said. Its the little things that matter. My guess is that barrel harmonics are changing ever so slightly due to higher or lower velocities due to differing bearing surfaces of the bullet causing the bullet to exit the barrel at a slightly different point of the barrels movement while firing.

[darkker]

To give you a visual, as it were...
This past Christmas I met-up with MZ5 and we did some pressure trace work. For a long time I've held the notion that Nosler is "copying" Sierra SMK's, with the CC. BC is the same, bearing surface measures the same, # of calibers in the nose, etc. Same load shows different pressures when all the user controlled variables are the same.
The difference is in the jacket alloy.

[barrel-nut]

+1 to everything darkker and stomp said

[PaddyD]

Thanks for the wisdom. I agree, now that I think about it, harmonics must be the issue. If the end of the barrel is tending to whip right at bullet exit then logically the bullet will be thrown right. I also think core composition must be a major factor as alloy obviously differs slightly from lot to lot. How a projectile compresses causes differences in pressures just as jump and charge do. When jump and charge are equall, bullet compression as it enters the bore must be a major factor in pressures which will affect barrel harmonics. There's so many variables in this science. Keeps it interesting for sure.
Again, thanks for your knowledge.