Encountered something new today. Worked up a load using the brass i have been using for about 250 rnds using Win brass and only neck sizing. Well found a load my rifle liked and figured it was time to use new brass. No pressure signs from old brass. No flattened primers at all at even near max loads? Well double checked new load again with old brass and was promising.Used new Win brass. Full length sized,deburred flash holes trimmed and prep like i usually do. So loaded new brass. Weighed powder ,the same AOL. Ran out of primers from previous box so opened new box and started loading. Same type primers WLR. Got to range and started out good but got worse. AFter 5 shots groups starting opening up and had flyers? Let gun cool to ambient temp between strings. Looked at brass after a couple of shots and found flattened primers? Also had 6 primers from old box left before i used new box. Loaded 28 rnds with load developed. 6 rnds did not have flattened primers but the rest did? The new brass felt different when seating bullets also. Old brass would start tight then just almost seem to just push the bullet in? I guess I should have annealed the old brass but did not and do not know how to do that properly. Read a couple of articles but I am still not sure? One article talked about overworking brass and making it hard. I guess i did just that. So what did I learn? When the bullets do not feel like the brass is holding them right and seating them becomes real easy get new brass or anneal old cases. Primers from different lots are different. Well I hope this info helps some other reloaders out there. What a frustrating day but i learned something in the end.