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butta9999 has it right, but my technique is slightly different. First of all don't load rounds under the factory OAL unless you have a very good reason such as a custom short throat. I you are unsure of the correct OAL, it's best to cross reference using different loading manuals. Second, if you are loading rounds into a magazine stick with the recommended OAL. If you want to hand feed each round ( my preference ) then load a few thousands off the lands.
I don't use a hacksaw. I make a dummy round ( no primer or powder ) neck sizing about .020. This will have enough grip to hold the bullet, but will allow it to slide back when the bolt is closed. Make sure the bullet is well past the normal OAL. After closing the bolt, eject the round ( carefully ) and take an accurate measurement, pull the bullet out and repeat, and maybe repeat again.
Then the most important part. You now only have a crude measurement. For a precise measurement make a new dummy round which is a few thousands longer than your new OAL, but this time size the neck so the bullet is tight. Take a candle ( a match will work ) and smoke the bullet. By that I mean blacken the bullet with a very light coat of soot. Run the round into the chamber and slowly close the bolt. Open the bolt and look at the bullet. If you see land marking, wipe off the soot, and put the round in your press. Seat the bullet .002 deeper and repeat the process. When you cannot see any land marks, or very faint marks subtract .002 from that OAL and that is what you will use only with your rifle and only with that specific type of bullet. If you change bullets or rifles, you must start over.
One more thing. You should start at the minimum recommended powder charge, because your pressures will be higher than normal seating the bullet so close to the lands.
Last....just a little clarification. A bullet is classified as a bullet until it leaves the muzzle. At that time it turns into a projectile, because it has been projected from the gun.
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