6.5mm 142's 'sweet spot'

[rbnhood]

Hi,
I'm new to the sight but have been doing a lot of reading. One thing I see many people comment on is that the sweet spot,speed) for the 6.5mm 139's and 142's is around 2900-3000 fps. Why are they less accurate at lower or higher speeds and how much less accurate are they? I realize each rifle is different, but I'm talking about generalizations. Thanks

 

[Rust]

Sweet spots exist because as the barrel vibrates up and down, it momentarily stops as it changes direction,the simplest explanation). This is called a null. If the velocity is such that the bullet exits the muzzle while the barrel is nulled the groups get tight. Generally accuracy is best where the barrel is nulled at the top of it's vibration.

Now there may be several velocities where the bullet will exit at a barrel null. Usually at least two. Since the nulls are pretty constant in a given rifle, loading to the middle of the null gives best results accuracy wise. At higher velocities the bullet spends less time in the barrel while the time span of the null remains constant,or nearly so) which means in effect the bullet sees a wider null. 2950 just happens to be where most 6.5 X 284 barrels null, there is a slower null arounf 2730 fps or so,varies a little from rifle to rifle) and there is a higher null but the loads would have to be pretty hot to get there and the 6.6 X 284 is already a barrel burner.

And this explains one reason precision loads are important, keeping the velocity spread minimized means the bullets all exit at the optimum time in the null. And differences in velocity certainly show up as vertical stringing at 1,000. Can't argue with gravity, it always works.

The 6.5 is a picky cartridge too, only a few powders work well with the 139-142 class of bullets. I used up a bit of barrel life with my first one proving that to myself. So 2950 it is.