The "M" dies have a plug that has rounded edges on the bottom, parallel sides up to a slight taper. If your case is a little undersize, the rounded edges will have to help "center" the case, or it will crush it. The RCBS expander dies have an undersized button with a taper that is quite a bit longer than the 'M' die. It will center the brass better and less chance for crushing or inverting a case mouth.
I took the expander plugs out of my 'M' dies and ground the tip of the button so they were more tapered and less apt to crush or invert the case mouth. After polishing the grind marks out, they work good.
Either work well if adjusted properly. You only want the case mouth open enough to accept the base of your bullet. If you open the case up too much several things will happen; first, it will not go into the bullet seat die. If it does go into the die, but still open too much, after about the 3rd firing, you will start to split cases at the mouth from over working the brass. Base of the bullet just fits in the opening, may even balance, and does not shave lead or copper. If shaving lead, open a little more. If bullet clearly fits and even starts to go inside the case, you are expanding too much. Sometimes a fine line adjustment. Mixed cases or untrimmed cases will become evident rather quickly, just like in the roll crimp die.
Steve
