Assuming all else is ok such as scope and mounts,
lapping and a good free float O ring (glass bed I like better) may help. From what I have heard others say the super light .243 are very heat sensitive. I would just do two shot groups and let the barrel cool before doing another. I am not sure if you are a reloader or not, but if you are not I am sure someone here can point you in a good direction with some factory ammo that works best for them . Best of Luck.
Norse