[Unclenick]

Mike,

A few thoughts:

First, just take your caliper and randomly sample ten or fifteen bullets of each kind and measure their lengths and calculate a standard deviation estimate for the population from that. The extreme spread of 95% should be within ±2 standard deviations. The result is the kind of COL variance you can expect if the seater step is pushing against the bullet ogive and not the tip.

When you seat a bullet with the flat ram by pushing on the tip, as your flat bullet seater stem is probably doing, you will get more consistent COL because COL's are measured to the tip, but if you had a gauge for the ogive position (which matters more to accuracy), you would probably find it was now worse.

You can often fudge that kind of gauge by using a spacer or you can also try resizing a case, flaring it (to avoid scratching your bullets), then inverting it over the nose of a finished round and measuring the distance from casehead to opposing casehead. Consistent measuring pressure is important as the cases are springy and often slightly out of round and are going be wedged into by the bullet. Zero on the first cartridge you measure to get an easy sense of the variation. You could also remove the seater stem from the die and use that, but then you're just asking to verify what you already know it does.

Take your seater stems and gently run the wide ends over a flat sharpening stone to level the raised edges of the stamped part numbers on top. You don't want these rocking on the aluminum stop plug during seating.

A bullet that goes in slightly cocked won't have the same COL as one that goes in straight. Check for the bulge in the case at the base of the bullet to be even all around. If it is, then you are fine. If not (if the sleeve alignment system isn't 100% perfect), use a Lyman M expander in place of the Hornady that puts a short step in the case before the flare. This ensures the bullet sits straight up on its way into the die. Midway and other common suppliers have them.