Like many things in reloading and casting what works for me may not for another.
The Redding G-RX die is a good example. I shoot a Glock 22, and reload with Lyman dies. The set I have will not size any bulge on 40 cases. I have a friend that ran the problematic case threw his Dillion dies with no problem. So all my 40 Brass now goes threw the G-RX Die.
Back to the Lyman M-die. I cast for four different Rifle calipers and use M-dies on each.
I also cast 7 different pistol rounds, but have never seen the need for the M-die with Lyman standard pistol dies. I have also used Lee dies. I do not like the floating inside expander in Lee Dies, but they two have always had the ability, to go deep enough to insert cast bullets without any shaving of Lead.
So what is the advantage of the M-Die with Straight wall Pistol cases, I have been Reading on other forums of the use and Modification of M-dies for use on Straight wall pistol cases.