Unless the base of that bullet is perfectly square to the axis of the bore
, gases begin to escape past one side of the base a tiny fraction of an instant before the rest.
This changes its position and the orientation of the bolt face to the center axis of the bore
If we're not loading directly down the centerline axis of the bore
, our off-axis pressure will combine with PIV to make our crosshairs hop off our point of aim.
Many have criticized this feature, claiming a short neck would not hold a bullet in proper alignment with the axis of the bore
With a 1911, the axis of the bore
sits lower to the hand than many of the more modern guns it competes with.
Since the Foster slug is almost as wide as it is long and considerably smaller than the bore to boot, it may or may not perfectly align itself with the axis of the bore
Usually, though, most of the weight (stock, receiver, magazine, lockwork, triggerguard and floorplate) is below the axis of the bore
. The result is muzzle rise, further increased when pressure of the stock against our shoulder keeps the rifle from moving back freely.
Good gunsmiths like Weigand carefully machine the crown square with the axis of the bore
and ensure that no little burrs or other deformities are present.
Mechanically driven parallel anvils extend simultaneously, and their consistent pressure results in more accurate readings and true alignment to the axis of the bore
. Linear accuracy to .00015" (0.004mm), capable of measuring from .08" (2mm) to 12" (300mm).
The bases of bullets seated with run out will not be perpendicular to the axis of the bore
, causing tipping upon exit.
Add to that the straight-line stock design, which puts the shooter's shoulder in direct alignment with both the recoil axis of the bore
and the linear axis of the gas-impingement, minimizing muzzle flip and subjective recoil.