Bullet run out is the cause of most reloaded rifle cartridge inaccuracies.
Through testing, we have found that Collet sizing a neck to .001 inch under the intended bullet diameter gives the optimum amount of bullet pull with the least amount of run-out. When using a collet Neck Sizing die, the rifle chamber acts as the sizing die, and tends to generate the most concentric running cases. The Collet Neck Sizing die just constricts the case neck a few thousands to again hold the bullet.
The Full length resizing die constricts the case neck much more. The expander then draws the case neck back out to a diameter again .001" smaller than bullet diameter, but it tends to stretch the thin side of the neck more than the thick. This tends to pull the case neck eccentric.
If you want to minimize run-out, stick to the Collet Neck Sizing Die.
