I had the exact same problem.The Lee crimp die is very picky about how it's adjusted and if it gets dirty.I used a different shell holder with mine and this problem showed up,ruining about 10 rounds for me.The locking rings Lee uses with the o-rings can be too variable as to how far they screw in and can do this if they get screwed down too tight.I also had the collet stick in the die a couple of times and crumple the shoulders as well.Crimping with a separate die may seem to be a simple step,but it can cause trouble.Be careful to make sure the die is properly adjusted and clean and lubricated with a little teflon based lube.I'm getting more and more convinced that crimping is an unnecessary step,even in a semi-auto 223.
