Nathan,
You should be able to get the runout down by several means. First, using the Lyman M die to put a small step in the case mouth for the bullet to sit upright in while you start it into the seating die will help a lot. Second, without doing that, I consistently get numbers in the sub-0.001" runout range with turned necks in the .308 Win using the Redding Competition Seater Die. Many folks report similar results from the Forster Bench Rest and Ultra Micrometer Seater Dies, both of which are less expensive and can be used with compressed loads, which the Redding cannot, but which can require a custom seater stem from Forster for some bullet shapes.
hounddawg said:
@ OP I have never really been able to find any verified testing to show that runouts matter. I am interested to see if Nick or Bart can come up with anything
A.A. Abbatiello wrote up an experiment in the 1960s for The American Rifleman (TAR) that is reprinted in the NRA book
Handloading, edited by Wm. C. Davis, page pp 86 (his illustrations) and 87 (text). He took samples from 42 lots of M72 30-06 National Match Ammunition (used the M1 Type FMJ match bullet) and measured the magnitude and direction of bullet tilt. His measuring method was similar to the Forster and Hornady types, with head and bullet nose supported (see my illustration of what the same runout looks like with different measuring setups below, with Abbatiello's being the first type shown). He found 0.008" of total indicated runout (0.004" of bullet tip-tilt off the cartridge axis) produced a 1 moa increase in group size.
With that bullet, tilt greater than about 0.0045" caused no additional dispersion, as entering the bore straightened tilt greater than that. He showed that orienting the high sides of random amounts of tilt at the same location around the clock in the chamber cuts that dispersion in half (in other words, it throws the bullets in the same direction consistently). He offers a theoretical calculation that agreed well with his result.
Abbatiello also references F. W. Mann's 1907 record of his experiments,
The Bullet's Flight from Powder to Target, a study of multiple firings that mentions the importance of the bullet's CG running true to the spiral path around the mean trajectory line for accuracy.
Abbatiello also references an experiment showing the same effect done by George L. Jacobson at Frankford Arsenal and published in TAR in the January 1960 issue that appears on page 20.
Another source is the late Harold R. Vaughn's book,
Rifle Accuracy Facts, which has a bullet cant test on page 133 showing the dispersion. Note that Vaughn's result is smaller than Abbatiello's. This is because he uses a shorter bullet for which an equal amount of tilt moves the CG off-center less than was the case with the M1 Type bullets loaded in the ammunition Abbatiello tested. Vaughn is an interesting authority because he was Head Aeroballistician at Sandia National Laboratories and well as having some good Boone and Crocket rankings, so he was both a ballistic scientist and a hunter with practical experience.
The bottom line is, the bullet choice, as well as the degree of tilt, determine how much dispersion effect you get.