Thread: .243 load development part deux

Originally Posted by gimp

If the bullet has an excessive free run or is poorly supported during that period it may tilt and become asymmetrically engraved as it travels down the bore - principle axis tilt. This imparts an angular velocity as it exits the muzzle - a sideways vector of variable size and direction. I.e. poor precision.

This can be mitigated by seating in that window of up to about .100" off the lands if possible, or at least as close as your mag allows if not possible, and by choosing to use cartridges with more modern chamber designs that have tighter tolerances in the neck/throat area.

A lot of people believe you can make small adjustments to seating depth within that window to "tune" a load. Noone has been willing to demonstrate it with evidence. I believed it for 15 years. When I started trying to test properly, I could not measure a detectable difference.

Seating depth also influences pressure and velocity.

I recommend never seating closer than .020 to the lands, you will see poor results with "just touching" the lands and .020 gives a comfortable margin away to cover for stacking tolerances

That makes sense and my question then is that if I understand correctly, distance to lands is a direct result of COAL adjustment, if I know my Max COAL (69.8mm) then this measurement puts my projectile just on the lands so my current COAL of 67.5mm puts me within the 0.100" - 0.030" window you mention?