In normal
applications, the throat is nothing more, nor less than an area in the barrel
immediately in front of the chamber neck that allows the bullet in a loaded
round to project out into the rifled part of the barrel. For this to be
possible, the rifling must be cut away. Thus, the throat is actually the groove
diameter of the barrel with the rifling cut away.
For best
accuracy, the groove diameter of the barrel should be enlarged very little if at
all, and then only enough to allow a bullet to fit in it. This insures that the
shank of the bullet is supported and guided as it is forced into the rifling. To do this, the area in front of the chamber neck must be a cylinder, not a
cone. If it is a cone, then only the loose fit of the case neck in the chamber
neck gives any alignment and support of the bullet as it enters the rifled part.
With at
least several thousandths of movement possible in any random direction at the
case neck, even if the bullet is centered in the rifling at its nose, as in
seating bullets out to the rifling, the base of the bullet can still deviate in
any direction out of alignment with the bore, and thus enters the rifling at an
angle, canted, cockeyed, or whatever term you relate to that means it did not go
in straight.
This
throws the bullet out of balance and accounts for much of the inaccuracy in all
barrels. This is not my dreamed up hypothesis. In addition to my own
observations, it is also confirmed by a nationally prominent scientist and
shooter, Harold Vaughn, in his highly technical book "Rifle Accuracy Facts." In
his book, Vaughn went so far as to measure the actual amount of bullet
dispersion at the target caused by specific angles of "cant" of the bullet as it
enters the rifling. The straighter the bullet enters the rifling, the greater its
potential for hitting in the same place as the bullets fired before and after
it.
The taper
cut on the ends of the rifling, also known as the "leade," helps center the
bullet as it enters the rifling, and the longer this angle is (presumably within
limits), generally the better the leade guides the bullet straight into the
rifling.
Regarding
the case neck in its role of aligning the bullet with the bore, the forces
involved pushing the bullet into the rifling are much greater than a thin little
brass tube, the case neck, can offset. Only in tight necked chambers with case
necks outside turned to give a loaded case neck diameter that is a tight fit in
the chamber neck can the case neck do anything to positively align the bullet
with the bore. But conversely, a closely fitted steel cylinder, the throat, in
front of the chamber neck CAN offset the effects of misalignment in the case
neck... and the chamber itself to a large degree, for that matter.