I shoot mainly single shots and that is a sign of too much pressure. I'd say you've hit your maximum powder charge for your load. I had a browning 1885 single shot in a 270. It would leave ejection marks and flatten primers, but the same load in my Ruger #1 had none of the effects. It could be you have maxed out your load or the Nef rifle has less tolerance for that much pressure. It didn't hurt your rifle, but if your sticking cases then I'd back off the powder charge. I really doubt you could cause enough pressure in that chamber to stress crack short of plugging the muzzle with something.