When you put the ram in the up position(shellplate down),just at normal rest position, is the shellplate balls properly in the detents? If yes, it is good.
When your bring the ram down(raise shellplate) about 1/4 way the shellplate will advance. When it does this does the shellplate balls drop down into the detents? If yes, the pawls are good, then it is something else. If your shellplate doesn't do as described then the pawls aren't adjusted right.
One other thing is that you say it does this and that only once in a while.Usually that means it is something with how you are operating the handle(or changing the speed ect..) USUALLY, not always though.
If what your saying is that the shellplate is trying to move at the same time the case is going into the die or the primer is coming up? Did I understand that part right? It should not move at all when the ram is within an inch of full up or down.
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