The case coming apart on extraction could be a head space issue. Maybe not with the chamber of the rifle but with the forming of the brass. Could be one or the other. If you had a way to measure the brass accurately and find the specs for that caliber you could see if there is variation. If you saved the brass check out if there appears to be a difference in the depth of the firing pin craters left in the primers that fired and those that didn't. Of course you can also try some ammo from a different mfg as Otto suggested.