I'm putting my money on excess headspace. The case is lacking the other signs of higher than normal pressures that have already been mentioned.
The "spiral" marks on the primer might indicate a semi-automatic rifle as well.
I have seen a number of older Remington semi-autos [740's, 742's] with very generous chambers, length-wise. [Read headspace, lol]
Ted, could you take a picture of the case lengthwise and post it? Thanks.
Regards, Dave.