if it was trimmed before sizing, then there is your bug. otherwise, were they "once fired" handloads that had been trimmed too short to begin with? or were they factory, and of what make if that is the case? when i neck size for a particular gun, i size one caliber of neck or until a sized case will fit in the chamber snugly (whichever requires more sizing) and with this method, i trim the cases only once, after first firing. there will be effectively no stretch this way, and i throw them away when i split a neck. dont know if this is any help for you at all.
