10 gauge resized shells will not chamber

[J.B. Books]

Not my words. "MEC Resize Ring for 600JR

The resize ring is a wear item located on the depriming station of the MEC 600 Junior reloaders. After repeated use, this part will wear and break or it may not fully resize hulls down to proper specs. For best resizing performance, this part should be replaced every so often, depending upon your reloading volume. If you are having trouble with resizing your hulls, you may want to use the MEC SuperSizer. This is a terrific alternative to the 600JR’s resize ring."

It's a six dollar part, if you are having trouble with guns not closing on reloaded shells and you don't have a MEC Super Sizer or Size Master it could be an inexpensive solution to the problem. As I said before I was surprised that mine was worn, but again we are talking a couple thousandths that can make a difference.