[Rocky Raab]

I'm like EWestbrook: I neither know nor care where the lands are. I begin by loading to the maximum length that will feed (or chamber) and seat deeper until I get best accuracy. Then I measure the case-head to bore-diameter (not groove diameter) datum line on the bullet. I can then load ALL bullets to that same overall cartridge length with no further testing.

Here's that datum line. This is a .25-caliber, so the datum line is the first point on the bullet where the diameter is .250". Measure from there to the case head and that is my "best seating depth" for that cartridge in that rifle.

 

[Rocky Raab]

The datum line trick allows you to repeat the best length without starting over every time you load a new kind of bullet. Simply scribe a line at the same diameter on the new bullet, and seat to the same base to datum distance. You'll be at the same "off lands" distance no matter what bullet nose shape. The only time it won't work is if you switch to an extremely long and pointy bullet that won't fit the magazine at the "best" seating depth.

 

[Rocky Raab]

Square One: what is the goal? If the goal is best accuracy at acceptable pressure, then it doesn't matter if the bullet is .0000001" off the lands, or a quarter-inch. As long as that is the most accurate spot in that rifle, what we are actually trying to do is reproduce that distance. It matters not what that distance is, as long as we know it and can put every bullet there.

 

[Rocky Raab]

You're welcome. It came to me as a blinding light when I was boning up on headspace. I read that they use a datum diameter on the shoulder to establish a set distance from the breechface, and suddenly realized that I could use the identical method to set bullet distance as well. Bone obvious - in hindsight!

 

[Rocky Raab]

Oh, for cheese. YES! Thanks. I'll go correct that. It should read (not groove diameter) as you point out.