I'm certainly no expert, but a couple of questions come to my mind.
Is the chamber new, never shot?
Is there carbon build up in the throat?
Are you measuring base to OGIVE or OAL?
What tools are you measuring with?
Did you use a cleaning rod to tap the bullet loose?
Did you remove extractor to prevent pulling bullet out of the case as you opened bolt?
You can partially size the neck to get a tighter fit to the bullet. Or you can neck size, seat the bullet long, paint it with marker and chamber. Look for scratches indicating contact with lands or tight throat.
Use a Hornaday or similar OAL gage. Takes practice to use it and get consistent results. Chamber has to be CLEAN, no carbon build up.
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