How to Run a Seating-Depth Test and Find Your Best CBTO

Once you've found your powder charge, seating depth is the knob that tightens the group. Here's how to test it and read what the target tells you.

Charge first, then seating depth

Find your powder charge first — that sets your velocity. Seating depth is the second pass: same charge, same everything, but you move the bullet in or out a few thousandths at a time and see which seating shoots the smallest, roundest group.

What CBTO is

CBTO is cartridge base to ogive — the length measured from the case head to a fixed point on the bullet's nose (the ogive), not the bullet tip. Tip length varies bullet to bullet; the ogive is what actually meets the rifling, so CBTO is the number that controls jump: how far the bullet travels before it hits the lands. Seat deeper, more jump; seat longer, less jump.

Running the test

Load a few rounds at each of several seating depths — for example CBTO at 2.250", 2.260", 2.270", 2.280" — keeping powder charge constant. Shoot a group at each. You're not watching velocity here; you're watching the group.

How to read it

• Group size — the obvious one, but a single number hides a lot.
• Mean radius — the average distance of each shot from the group's center. It's a steadier measure of a load's true precision than extreme spread, because one flier doesn't dominate it.
• Vertical spread — at distance, vertical is what costs you. A seating depth that stacks shots vertically is worse than one with the same group size spread side to side.

You're looking for a seating depth where the group shrinks and the vertical settles down — ideally a small range of depths that all shoot well, so you've got room to seat consistently.

Picking your seating depth

Choose the depth in the middle of the range that shot tight and flat, confirm it, and record the CBTO so you can reproduce it. That number plus your charge is your load.