JohnK3, boy, now you are dating me as an old phart. I still have, and use, my MEC 600 jr press to this day. I have loaded many, many thousands of 12 ga shotshells on it and have finally wore out my sizing die. I called Mayville Engineering about two weeks ago and they are sending me a new sizer die. I love that old machine and will not let it go. I have every powder and charging bushing ever made for that old timer.
As for overpressure, the only other thing I can think of is DO NOT DEVIATE from the load book data and parts used. I have always used Winchester AA hulls and have never had a problem. I did have one shell I did not drop a powder charge into and that was the only mistake I ever made. One shell out of literally thousands.
Went tinkle, tinkle tinkle, down the barrel. No harm done. Made sure barrel was not obstructed, racked in a new round and away I went. Haven't ever had another one!
Try the 600 jr. It's a lot of fun. I have the auto primer feed for mine. A must in my consideration!