blarby - that's a result of neck tension being different.
The brass thickness on the neck can vary by as much (possibly more) as .005" on commercial brass.
When you size a neck, ALL necks are sized to the same OUTER dimension, then the expander pushes them back OUT again on the downstroke. If neck thickness isn't uniform, the brass springs back differently (the math behind the metallurgy is beyond me, but it's not linear). The net result is an interior diameter that can vary by as much as .002.
When you go to seat the round, you get the differences in interior diameter of the neck, PLUS the variations in tension caused by the varying THICKNESS of the brass in the neck.
The net result is bullets which have WIDELY inconsistent seating tension.
Not a huge deal for the casual reloader - the pressure in a round is massive and will get the bullets scooting out of there every time at roughly the same velocity.
But for accuracy,
roughly isn't good enough.
Neck turning - if done properly, with the correct sequence of steps (which includes correctly sizing both pre-and post turning) - will eliminate ALL vertical stringing on your groups and dramatically lower the standard deviation of your velocities.
(Your groups will be nice and round, and shrink.)
Doesn't matter so much at 100 yards, but when you start getting out 300, 400+, those velocity differences compound with every OTHER factor affecting the round's trajectory. It changes your wind characteristics as ALL rounds will be equally affected by wind - slower rounds drift more than faster ones. It'll change the spin drift characteristics, which ALSO compounds with wind resistance as rounds "climb" in to the wind and "drop" away from the wind, with respect to their turn direction. It will also neutralize differences caused by air density, as all rounds will slow the same rate (slower rounds tend to slow faster than faster rounds slow, inertia 101).
So having as tight of a muzzle velocity range as possible, is crucial to accuracy.
