Here is an easy way to do it. it will get you close enough to start.
1) take a once fired brass that will chamber in your rifle. (not resized, just fired, a bullet should be able to fall right into the shell.
2) push the neck of the brass lightly (sideways) on your workbench. What you want to do is just slightly bend a small spot on the neck, so a bullet will have the slightest bit of tension on it as you slide it in.
3) Take a black felt tip marker and color a bullet completely black
4) Start the bullet in the once fired brass and gently chamber it in your action. The bullet will touch the lands and then be pushed back into the cartridge, leaving a scratch on the ink.
5) gently take it out of the action and you will see a scratch on the bullet from the small dent you put in the neck.
6) put the bullet back in the cartridge right up to the end of the scratch and carefully measure the length. Write the length down and do it 2-3 more times, until you are sure of the length. That length bullet will put you into the lands a few thousandths, so you want to make them a little shorter than that bullet. Seat a bullet 10 thousandths deeper than that and you should be good for starting loads. On the first round that you make that length, blacken the bullet by holding it over a candle flame(just the bullet part) and gently chamber it in your rifle (safety on and pointed safely). You should NOT see land marks on that bullet, but you will be within a few thousandths of the lands. You can use this last method to find the exact length to just touch the lands if you wish, but its normally good enough to get a few thousandths off and see how it shoots.
It isn't a very technical method, but poor boys learn to do with what they have ;), this is one of the methods I still use from time to time when I'm in a hurry with a new rifle.
If you don't have a once fired brass, there are other methods. Let us know if that is the case.
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