From the Hornady instruction manual for their O.A.L. Gauges and what they recommend.
USE COMMON SENSE WHEN SELECTING A BULLET SEATING DEPTH. We recommend a free-travel (clearance) of .020" to .040" between the bullet and rifling lands for hunting rounds. In our experience, excessive bullet free-travel (over .040") will most often result in a loss of accuracy. However, each gun will have its own preference, and it is up to the handloader to determine the optimum bullet free-travel by range testing the
loads. There are no short cuts in this area. However, once the optimum free-travel is determined with one brand of bullets, this same free-travel dimension will most often be preferred with other bullet brands or weights. Remember, if other bullet models or weights are chosen, they will have differently shaped ogives, which will require that you repeat the O.A.L. Gauge process with the new bullet. While not absolute, there are several reasons for our free-travel recommendation: (1) A more uniform chamber pressure is produced to start the bullet down the bore. This reduces pressure “spikes” resulting from an interference fit. The result is a more uniform velocity which produces accuracy. (2) As the bullet is aligned in close proximity to the rifling, it will stabilize quickly as it exits the bore. (3) Safety and reliability; a .020" to .040" free-travel produces reliable feeding and extraction.