Reloading .303 British : H4895

The goal is to have just enough "jump" into the rifling leade that the bullet will separate from the case, but not have the ability to "tilt" in the slightest. Because of that OAL is a substitute measurement thats easy to measure & replicate from bullet to bullet type & weight.

What you are actually trying to do is have the part of the bullet's ogive that matches the land-to-land diameter exactly "xx" thou from the point of engagement. Then you measure the resulting OAL just because its easy to set up a seating die that way & measure it with a simple caliper.

Because of that bullet shape comes into play.

That's also why the Hornady tool has all those bushings, so you can measure from that diameter instead of the tip!