ORIGINAL: James B
Flattened primers can mean excessive pressure but it can also be caused by poor head spacing or loads that are to light and detonate instead of burning the powder. Primers backing out can be loose primer pockets or again, excess pressure. You are right to assume that it does not automatically mean high pressure. However most of the cases Of flattened primers I have had have been excess pressure.
As a general rule, flattened primers are NOT a reliable indication of EXCESSIVE pressures - the causes of flattened primers are many and varied! RELIABLE indicators of excessive pressures include bolt face/ejector slot marks on the case head, difficult bolt opening, and primer pockets that expand to the extent that it becomes easy to seat new primers (OR WORSE). Primer pockets don't WEAR bigger just from the friction of punching out and installing new primers. They expand due to too-high pressures!