When the Dillon's 550's first came into use dies on hand would work. However, the Lee dies at the time turned up short for the tool head. The idea came from many directions is to put that lock ring on the bottom side of the tool head. This length issue went away with the current generation on Lee dies. I use Lee dies for 357 SIG with Dillon crimp die. Otherwise, I also have Lyman and RCBS dies in my tool heads with no problem. Honestly, I have a strong preference for Dillon dies.