i use a stoney point oal bullet gauge with a sinclair international bullet comperator and a set of calipers.however if a gun is to be used for hunting, the limiting factor may usually be the magazine size of the rifle. i have several rifles which require me to seat my bullets 30 to 50 thousanths off the lands to account for this.these measurements are taken from the bullet ogives.every barrel has a sweet spot as to where it will shoot best in regards to bullet seating depth. the only way to achieve the best seating depth is to shoot it in i.e, shoot groups of 3 or 5 shots with different seating depths until the target shows you what the gun likes best. just remember to start with a clean and cool barrel each time and to repeat a successful combination to make sure it's not a fluke. once you find the right seating depth you will usually be able to shoot your first shot out of a clean barrel that will group with your consecutive shots. this is one of the greatest advantages to reloading as it is repeatable accuracy. you are starting from the same place everytime.if i'm pulling a gun out of storage i run several dry patches down the bore and i'm good to go.

FWIW I use a stoney and get an average after about 10 readings. I take that and regardless of if its correct or not I use it to decide my depth. IE subtract or add XY amount to that figure and start testing.

Then I end up with whatever is most accurate and make a dummy round of that with a bullet. You can bet your throat will get longer and not shorter so when accuracy dives a bit, I"ll go back to the dummy round and add a bit to OAL.

Its not scientific, but it prevents me from wondering which figure is right etc..... But then again it probably works best for me since in my match ammo I tend to jam the bullets almost all the time so I simple add to the figure and am happy.

Jeff