When I check neck concentricity on a fired (unresized ) cartridge, the tollerances come out better than after neck resizng. Is this to be expected with standard dies?

Pretty much. I've about given up on neck sizing, since I got a gage to measure it. Get better results with full-length sizing dies, adjusted for the rifle.

Don't have any real high-dollar neck sizing dies so maybe my experience is biased.

Welcome to the forum!

What type of dies are you using? Regular full length adjusted for partial sizing, or a neck sizing die? I wouldn't call it common to lose much concentricity through sizing, but I've typically only checked runout if there was a problem with accuracy that couldn't be otherwise explained.

Some neck size dies are universal to caliber, or don't have a very good fit to the case body, I think these could lead problems to a greater extent than other die types...or you could just have a bad size die. If there is a bunch of gunk built up in the slot in your press ram and it doesn't allow the shellholder to slide all the way in it could cause problems also, or the same problem in the shellholder itself.

My best guesstimate is that when a cartridge is fired, the expanded case retains much of the chambers dimensions and it's understood that chambers are pretty concentric. It's when the case is neck sized that many variables come into play. Is the case rim concentric, any burrs on the rim, foreign matter in the shell holder, case positioned correctly in the shell holder, expander plug concentric in the die, and so on. All these factors resulting in a neck sized case not being as concentric as a fired one. As Mike says, a full length sized case tends to be more concentric. The full length sizer die sizes the entire length of the case whereas the neck sizer does not.

Even a full length die can produce crooked necks if the neck section of the die isn't concentric or out of line with the case body section. That can happen if the die is moved from one lathe to another between operations. RCBS does it this way, and I have a .222 die that will produce .012" max runout. Try turning your case 180° and running it in again. That might cancel the runout.

Another source of runout is brass that's thicker on one side than the other, but these usually come out of the chamber crooked.

Bye
Jack

Thanks for the thoughtful insights. I tried this forum because the folks I shoot with are not as particular as I am about their loading.
For those results I used an RCBS (.270) neck resizing die. I will take it apart and check for trueness on the decapping/neck expander rod. I try not to full length resize knowing the value of a fired fitted cartridge. I'll get match grade dies if I have to. I've been warned about opening the concentricity "can of worms"...finding problems that would not have otherwise caused worry. Still, sitting at the reloading bench ain't a bad way to spend some time. Thanks again.