I've been manufacturing bullet seating depth gauges for the past few years. Prior to putting them on the market, extensive testing was done in labs to determine the best seating depth and more importantly, why.
Why? Guns made in the last few years have deeper throats (mistakenly called free bore) due to looser SAAMI specs. Most gun manufacturers have taken advantage of this to reduce the cost of production. That means the consumer isn't getting the nice snug chambers that we used to.
The only way to counter this is with proper bullet seating depth. What happens inside the chamber when a round is fired has been somewhat of a mystery until new piezo pressure testing sensors coupled to computers became available. When the bullet is forced from the case under extreme pressure, it strikes the bore, and if not perfectly centered, will cause damage to one side the bullet (the section that hits the bore first). After the deformed bullet leaves the muzzle, it is slightly out-of-balance. Down range, the deformed bullet whiffles and opens up the group. If bullets can be started into the bore with minimal damage, the bullet will maintain balance and groups will tighten.
The labs conclude that bullets in bottleneck cases should be seated .010" off the lands. This minimizes bullet strike damage. Seating depth effects the chamber pressure, powder burn, and velocity. Many reloaders find a powder charge that works well then adjust seating depth. This is backwards. The seating depth should be set first, then the powder charge adjusted for optimum accuracy. By adjusting seating depth after a charge is set, the powder's burn characteristics change and you chase your tail. In magazine fed guns, the bullet may "want" to be seated out farther than the magazine will accommodate.
rmtaylor, Mike answered your question. This applies to bottle neck cartridges only. Straight wall rifle or pistol cartridges are roll or taper crimped and do not follow this convention.
See:
http://www.cactustactical.com/reloading.html
