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I've reloaded for a while but have never really got into really accurate stuff. I am reloading 6.5CM 120gr GMX's right now. Hornady a book says that the oal for that bullet is 2.71 but the max for the 6.5CM is 2.8. How much difference does it make if the bullet is seated above the 2.71 but not above the 2.8?
OAL is given in manuals so that a loaded round can function in any standard chamber for the cartridge you are loading for. You will fond that some rifles shoot better with the bullet seated closer to, or even touching the rifling in the barrel, and some shoot better with some jump between the bullet and the rifling. You need to be concerned about the cartridge fitting in the magazine in some cases when you load longer than the max listed OAL. You also want to start measuring from the ogive of the bullet to get more accurate seating depth. Finally, with some bullets, particularly ones that are on the light side for a particular round, you may not be able to seat them out long enough to touch the rifling because the bullet will come out of the case neck. Also, be sure and work up slowly on your loads when you are seating them to touch the rifling as pressures increase when you do that.