Precision is all about uniformity and alignment. First of all is the uniformity of your primers, powder charges, and bullets (in terms of weight, bearing surface, and diameter). If your cases are straight, and your bullets are seated straight, and your chamber is straight, and the leade is straight, and your barrel is uniformly bored and rifled, you should do well. The amount of variation in all of the above is what makes all of it a test-and-see endeavor.
An interesting example of precision when it comes to outputs of the system are the 6.5 CM and the 6.5 PRC. The reamer spec for both of these only allows for a throat only half a thousandth of an inch over nominal bullet diameter, and the leade on both is cut to a target rifle standard of 1.5 degrees. These two things probably have the most to do with precision in shooting, once everything else is relatively consistent, as they cause the bullet to enter the barrel pretty straight, thus allowing it to travel down the barrel pretty straight.
