I am new to reloading.  I have a Hornady LNL AP progressive press.  I have loaded 40 S&W and 45 ACP so far.  I use Hornady's custom carbide 3 die set for both.  I notice after I load a batch of about 10 cartridges that the COL of each one is different.  I might get one or two that are the depth I set the seating die for.  If I run each one through the seating die again I get some correct and some even shorter than the original one I set the die for.  Anyone else have this same experience?  If so were you able to find a solution?  I'm a little disappointed right now with my results.  By the way the lengths can differ by up to 8 thousandths.

Soms variation is normal on a progressive due to flex on the toolhead under load.
Another source of error is the seating die punch/stem not matching the nose of the bullet.
Eg. Trying to seat a round nose bullet with a swc punch is not a good idea.
Those would be the 2 major factors.

it might also depend on the type of seating die that you use.  if the die also puts a crimp on it, then that may factor into the COL variation.

if your die does a seat/crimp at the same time, you may be able to back the die out enough so it only seats (and doesn't crimp).  if you still need the crimp, or you need to take out the case flare, then you can always crimp in a separate station.

Measure everything and see how it compares. Sort the brass and bullets by length. I was having troubles with one load until I realized there was a lot of variety in the length of bullets I bought.