The tighter chamber will have very little affect on pressure. The throat or possibly you just have a tight barrel. Is there a huge carbon ring that is causing problems? I would probably check the chamber with a known reamer and see if it cleans up. I had a 708 that was doing that. We ran a different reamer in and my issues went away. I find that if I use a barrel from the same guy and ream with the same reamer, I can interchange brass if I want to. I can control the headspace completely. I also full length resize every time with rifles that going into the field.
