About those primers!. Primers cannot "back out" unless there is excessive headspace, either in the chamber itself or with the ammunition used (shoulders pushed back on rimless ctgs). Low pressure, usually under about 43,000psi will make the excessive headspace visible by the slightly protruding primers. Simply firing a primed case, no powder or bullet will usually shot this trait of sloppy headspace.

If the pressure is higher, there is a tendency for the case to slam back against the bolt face which will re-seat the primer. This re-seating of the primer usually causes excessively flattened primers, misleading the 'shooter' to think the ammunition has excessive pressure, when the pressure could be less than 50,000psi.