I fired some new Rem .410 3" loads in my Spanish double, and had primers cratered, pushed into the breech, making it hard to open. Fired some older Winchester, no problem. Has anybody else noticed anything strange with Rem shotshells lately? Maybe soft primers, or high pressures?

I recently shot a flat of .410 Remington shells in #7.5 without any issues. I shot the shells in three differnent .410s (Beretta O/U, Savage SxS, and a Grulla SxS).

Maybe it's not chambered for 3" shells, which would explain a lot.

Stepchild

No, it's definitely a 3" chamber. Note that the Win shells show a normal primer hit, no cratering. Rem sent me two boxes as replacements, same cratering. Makes it hard to open, when primers are pushed into the breech holes. Very weird...

xs, have you checked your firing pins for "xs" protrusion? This could 'splain the effect based on differences in primer hardness between Remington and Winchester.

I'd ask them to "fess up" (if that is the correct expression) as to if the cases are TRUE .410" cases or, in fact, European made cases that are not TRUE .410" cases.

Some older British guns also have this issue with these European cases in that they won't extract properly as the head and rim size is not actually correct.

These European cases may be marked as "12 M/M"

It may be worth an ask.

Or see here: http://www.fourten.org.uk/chamberdimensions410.pdf