Well I have to say in a 243 and just talking 100g bullets, you have a plethera of bullets to try before giving up...
Some guns just dont like boat tails, others plain bases. Then you have OAL to play with , some dont respond till you get with in a couple thousandths of the leade others dont care... some really respond to a crimp or perish the thought "factory"crim for a LFC die. THEN there is at least 7-10 really good powdrs to try....
The very first "NEW"rifle I bought myself was a brand new Winchester Feather weight Deluxe in 243 with a brand new Leupold Vari X 2 3x9x40 scope. I still think this is one of the best looking factory rifles one can buy. BUT it would not shoot, I mean would NOT shoot, 4-6"groups with every factory loading and maybe 10 different powders and a half dozen bullets... FINEALY after re crowning, bedding the action, tuning the action and trigger I was able to find that a 90g Speer behind a near max loading of Brigadieer 4351 Powder and seating the bullet just off the rifleing netted me, round, 2"100 yard groups. Its not great accuracy, but I'm telling you I worked HARD at the bench making shure it was not me or my loads causing the accuracy problems. Its the best this gun would shoot.
What powders have you tried? Have you tried a boat tailed bullet or all plain bases? What about seating depths, have you checked max OAL for your rifle and set up loadings with in .005-.007 off that number?
Good luck,
CW