So heres the deal:

I have 20 Hornady 300 win mag cases that i have been neck sizing over and over. They are now making it hard to close the bolt, no problem ejecting, just chambering.
I bought the brass as once fired, full length sized them once, and i have neck sized using the Lee Neck Collet 4 times now. So they have been fired a total of 6 times. They are not over the max length, and brass still seems to be in very good shape.
So what should i do now? Full length size and start over? I have herd whisperings in the dark about "bumping the shoulder back" what is this? How would i do it? The tools i have are an RCBS Full length sizing set and a Lee Neck Collet die.

Any pointers or words of advise would be appreciated,
Thanks - Leo