A guy brought a Remington 700 Tactical 308 to me about three weeks ago with exact same problem.
I ran headspace gage in it and it would not shut on a GO Gage. I pulled out the bore scope for a looksee and found the chamber thoroughly rusted. Upon questioning I learned he had taken it hunting in the rain, came home and put rifle with chambered round in gun cabinet and forgot about it.
Next time he went to shoot the round chambered fine but when fired and bolt manipulated it was froze up.
I pulled the barrel and first and cut threads and tried to put him a min chamber in with tighter neck and the Remington chamber was jumbo so I wound up putting him a SAAMI spec chamber it it and cleaned all the body and most all the neck removing the pits.
Put barrel back on and chambered it tighter on headspace and he went out and shot it and it did fine. He was very lucky as there was no pitting up in rifling. Otherwise he would have been out 350.00 for a barrel job.
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