I want to make sure each Round I make has a consistent OAL.
consistent overall length (OAL) or consistent base to ogive (BTO) ? Your seating die is hitting somewhere on the ogive so you will always get the same BTO at the point where the stem contacts your bullets. To get a consistent OAL you would have to modify the seating stem so it is hitting on the meplat of the bullet. To get consistent BTO measured by the comparator the seating stem would have to contact the bullet exactly at the same point on the ogive where the comparator is contacting the bullet or the bullets would have to have perfectly uniform same ogive measurements.
This is where reloaders drive themselves nuts. Unless the bullet ogives are perfectly uniform the seating stem will not be hitting the same spot on the ogive as the comparator will and the base of the bullet to the meplat will vary also. Therefore any variation in the ogive will result in a variation of BTO measurements when measured with a comparator and any variation in base to meplat on the bullet will result in base of the bullet to meplat differences unless the seating stem is contacting the meplat instead of the ogive which would result in BTO differences on the seated rounds. (OAL).
Now we measure each unseated bullet with our comparator and make sure the seating stem does not contact the meplat. Problem solved right ? Nope becasue unless the ogives are perfectly uniform or the comparator is contacting the bullet exactly where the seating stem contacts the bullet any differences in the ogives will translate into slightly different measurements on the finished rounds
What
will always be consistent is the base of the finished round to the
point on the ogive where the seating stem contacts the bullet which is why measuring the base of the bullet to the point on the ogive contacted by the comparator is a waste of time. That is unless your bullets are
exactly the same which you have already stated is not true.
Simple right? I really need a whiteboard and a lectern to explain this
edit
imagine this exaggerated example
you have two bullets one has a VLD ogive the other a round nose. Both have the same base to meplat or overall length. The seating stem will contact both bullets at the same place on the ogive's diameter however that will not be the same point measuring from the base of the bullet. The VLD's meplat will will be extending farther into into the concave portion of the seating stem
The good news is most real world tests show a variance of .003 jump to have a minimal affect on accuracy