I' ll bet iowabuckslayer has it right. I' ve had this problem before and it took me about 7 years to figure it out. This question comes up periodocally on these forums and 90% of the time this is the problem.

The problem is with the way the bullet seating die is adjusted. Most standard seating dies have a crimp feature built in, they' re designed to crimp the bullet into place at the top of the ram stroke. The problem with that is that most bullets we load don' t have cannelures on them, and the ones that do often aren' t in the correct place for crimping after we play around with our bullet seating depth. What happens is that if your seating die is adjusted down to where it hits your shellholder at the top of the stroke then the case is shoved far enough into the die to hit the little crimp " lips" as I call them. These will try to crimp the mouth of the case into the bullet. If there isn' t a cannelure there to allow some give in the system then it puts a lot of stress on the case and bullet. What happens is that something has to give and the typical place for this to happen is right at the top of the case where it meets the shoulder, the case will get a very slight buckle in it, not enough to be obvious but enough to make the round not chamber or chamber hard. If you run your fingernail over a case that' s been buckled you can feel the bulge at the top of the case. You might not even be able to see it but you can feel it.

The fix for this is to screw the bullet seating die out until there' s about a 1/16" gap between the shellholder and the bottom of the die when the ram is at the top of it' s stroke. Now use the seating stem adjustment to set your bullet seating depth. If the shellholder isn' t hitting the bottom of the die then it won' t try to crimp. This might not be your problem, but I' d take a $100 bet that it is.