Being a 243 Win. owner and shooter and tinkering on many since 1965. There is a couple things I have seen no reference to. (1.) Have you chronographed your rifle when shooting? I have seen 243's that love certain primers. Cleaned a bad full channel glass bedding out and left good action bedding in a 243 Win. The guy said it shot less than 1" groups before the bad bedding. I used CCI standard primers in a load the gun shot 1 1/4 " at 100 yds. I changed to Remington standard primers. Urica, A dime covered 5 shots at 100yds.
(2.) A weak firing pin spring can cause the best rifle to go FLOP. Any sheared sharp edge on the firing pin crater edge is a sure sign. And easy to check with use of a scale.
(3.) What type die are you using full length, neck size?
I have seen 243 that would did not like anything but neck sizing, Shot factory ammo like buckshot.
If it is a bad barrel this may be the only cure, "Replace It". Good Luck in your quest.