Date: Oct 26 2005
The first criteria of seating bullets is that the loaded round should fit and work in the firearm. If the bullet is seated too far out to fully chamber, cycle through the magazine, or (in a revolver) to allow the cylinder to turn, then the bullet needs to be seated deeper.
In many commercial rounds, the maximum overall length (OAL) is listed in loading manuals. This is the conventional maximum length that will function in standard magazines or cylinders. The bullet, however, does not necessarily have to be seated out that far. Shorter seating will often promote better feeding from the magazine, particularly with roundnosed bullets.
Second, the bullet should be seated as close to the start of the rifling as possible, given the limits of magazine function. This helps prevent the bullet from turning slightly sideways while it travels from the case mouth into the rifling, which tends to promote better accuracy.
With jacketed bullets, the finest accuracy is often found with the bullet seated just slightly "off" the lands. I normally start at about .03 inch, then adjust from there if seating depth might be a factor in accuracy. (In hunting rifles, jacketed bullets should not be seated so they actually contact the lands. This tends to create higher pressures – and a bullet so seated can actually be gripped by the rifling, then pull out of the case when the round is ejected. This creates a real mess with spilled powder inside actions.)
Benchrest and black powder silhouette shooters, on the other hand, often seat bullets "into the lands." Normally there is no problem ejecting such an unfired leadbullet round, and benchrest shooters rarely have to eject a case. (If they do, they often use a cleaning rod to push the round out, rather than risk spilling powders inside their very tight rifle actions.)
Some rifles will respond better to bullets seated well off the lands. This usually occurs when the throat (the unrifled segment of the barrel between the case mouth and the rifling) is only slightly above bullet diameter. This is common in custom, match-grade chambers but can occur in some mass-produced rifles as well, so the "rule" that bullets should be seated close to the lands isn't iron-clad.