If they are getting loose, its more than likely from work hardening of the brass. Try annealing the necks.
BINGO!!!!
I've seen this many times before includeing with the .338-06. I don't like annealing case necks so those thathave loosetension go to the scrap pile.... (soon they will crack)
I ALWAYS keeps my lots of brass seperate and shoot them as groups keeping track of how many times they have been fired ect....
DM