Short Cases, Lee FCD, and bullet depth?

[Netzapper]

I've noticed that some cases are shorter than others by several thousandths of an inch. This is even between cases from the same box of factory ammo. The median difference seems to be about .006".

As a result, using the Factory Crimp Die results in different widths of crimps on each of the cases.

And now that I think about it, does this affect the bullet seat depth? Or is the seat depth more an issue of how far the bullet is from the case head (OAL - bullet_length), and not how far the neck sticks up over it?

How big of a deal is this? I mean, if I were a competitive benchrest shooter, would I notice the difference? Not that I am competitive, but I am a perfectionist.

If it is a big deal, how do I correct it? Fireform the brass then cut them all down to the same length?

[Unclenick]

The way SAAMI specs cases and chambers is cases are maximums dimensions with minus tolerances and chambers are minimum dimensions with plus tolerances. In case length, it is almost always the maximum length minus 0.020". Trim lengths are almost always given as the maximum length minus 0.010" just to put your trim in the middle of the tolerance.

If you want consistent crimp pressure, you do best to trim all your cases to match the shortest one. This will affect seating depth for bullets with crimp grooves and cannelures, but 0.020" variance in seating depth causes pressure changes that are generally lost in the noise of powder charge and ignition variance, so it isn't a concern.

Seating depth = case length + bullet length - COL.

If you use a standard length for making that calculation, then the distance from the bullet base to the inner case head will be constant, and with that should come constant pressure, within the limits of the charge and ignition noise.