Check the adjustment of your sizing die. I t may not be hitting the ram hard enough. If that is the case the cartridge will not be sized quite right and will be hard to chamber. 243 seems to be pretty particular on that point. The die should touch the ram and then be set down about 1/4 turn more to correctly full length size the cartridges. I have never had this problem with 30-06 or 270 but just recently experienced it with a new Tikka T3 hunter and that is how I solved the problem.