The .243 can be finicky at times. I had a Remington 742 that was very accurate with Federal, Winchester, Remington, and handloads. Normally 1" to 1/2" groups at 100 yards. I tried some Lapua (Norway or Sweden mfg.) when stationed in Germany and they all stuck! I would shoot and then half to pull on the bolt to get the brass out.
I have an NEF in .243 that began shooting all over the paper. It took awhile to break it in. Shoots fine now. My other .243 is a Remington Model 7. I've only used Federals so far with acceptable groups - 1 1/2" to 2" groups thus far. I have some Remington and some hand loads waiting for my next trip.