While I am new here, great forum by the way, I'm not new to guns or hand-loading.
I've been hand-loading rifles and pistols for 30+ years and about half is for tailored for my bolt guns. The best accuracy loads I've been able to achieve is when the ogive of the bullet is "just touching" or about 0.005 off the lands. By doing this it allows the bullet to move on initial firing and ensures concentricity of the projectile and bore.
I have tools that allow me to measure the over all cartridge length for that particular bullet as it touches the lands and then I back off the 0.005 I need for the gap.
This is the amount of "grip" the brass neck applies to the outside surface of the bullet. Typically, a light tension force provides the optimum condition for the best accuracy. Keep in mind that this is for bolt guns and not semi auto's
like or AR 15.
Semi-auto's need more neck tension to keel the bullet tips from moving during the bolt cycling bringing a fresh round to the chamber. Believe it or not, this is somewhat of a violent action which supports a heavier hold on the bullet.
Sizing & Barrel Chambers
Standard bottleneck rifle dies are machined to restore a fired case to it's unfired dimensions. The sizing die is machined using reamers similar to those used to ream the barrel chamber and in fact, customer rifles are chambered using specific reamers and these reamers are also used to machine the sizing die!
I use bushing neck sizing dies for 3 of my bolt guns. These dies use specially jig ground carbide inserts that allow me to adjust the neck to achieve tension as low as 0.0005.... Bolt Guns Only!
Once the case is fully sized, it's retracted and the neck of the case slides over the expander button ensuring the neck ID meets specifications established by Sporting Arms and Ammunition Manufacturers Institute or SAAMI. This dimension is really all that's necessary to secure the bullet in place.
A Cannelure is a groove placed at some location on the circumference of the bullet. These groves are found on nearly all of the large heavy cartridges due to their punishing recoil. All the military rounds I'm familiar with incorporate them as well to insure their dimensional consistency and reliability under the most adverse conditions.
Military rifles are machined to certain dimensions in order for them to use any ammunition designed for that chamber. Their parts are also designed so that their interchangeable as well, IE bolt from rifle A fits in rifle B without incident.
Bullets with cannelures need not be crimped just because they're there. Run them up in the seating die and adjust the die so that the crimp feature does not engage the case mouth.
Hmmm, I think I may have gotten off track a bit...