All that stuff about datum lines and case body length is all well and good, but it means nothing unless the case is too long for it to fit in the chamber. Shortness in that regard has no bearing whatsoever as, like was mentioned, these cases' headspace is determined by rim thickness- a too short case body will simply blow out to fill the chamber. Has anybody with "primer protrusion" issues measured their rim thicknesses? Has anybody actually employed a proper set of headspace gauges to determine the headspace of the suspect rifle? How about measured chamber pressure, rather than guesstimated pressure?
Primer protrusion is a result of two things: excess headspace (actual headspace, as measured for a rimmed cartridge, not as measured for a rimless one), and lower than optimal pressure upon discharge. Too low pressure will be insufficient to drive the case firmly back against the bolt face (because of its "grippage" of the chamber wall, and pressure not being sufficient to fully overcome that) but regardless is enough to back the primer out a tad. A well polished chamber will alleviate some of that by reducing the "grippage" of the brass against the chamber wall. But, in the end it doesn't really matter. Those of us who have fired thousands and thousands of reduced loads in rifles have long noted this phenomenon, and shrugged it off.
Guesstimating chamber pressure by measuring primer protrusion is as much voodoo as is observing primer flattening and measuring case head expansion- Ken Waters notwithstanding. He wasn't a god and much of what he preached 40 years ago has since been disproved.
