Do I have a Brass or Chamber problem?

[Red Dot]

I am loading for a .223, 12", Contender, Custom Shop.
I am loading a starting charge of 20.5 gr. H4198 using 50 gr. Berger standard, just off the lands.
With cases fullsized, prepped, charged and bullets installed. Off to the range. Winchester Brass.

Today I shot 20 rounds, 4 five shot strings over 30 minutes.
Here is what I find: average groups w/ 2x20 Leupold
Fullsized measurement at base .374 Unfired
After firing the cases measured .375 to .377 (correction .3757).

Only three of the fired cases would close properly, when fitted in the chamber.
The rest went from hard to very hard to close.

It seems as though they grew bigger as the barrel heated up.

I have been full sizing because of this problem and I thought I would look into it.

Do I have a brass or chamber problem? Your thoughts please.
Thanks Jerry