Be carefull about having your shell plate touch your reloading die. According to Dillon this can cause damage to the die if you are using carbide dies which I assume you are. I have the 550b and I had the same problem but with 40s. I found out that the sizing ring in the die was of the wrong size and was told to destroy the die by Dillon so no one could ever use it again and they sent me a new one at no cost. I load thousands of 9mm a year on my 550b and of all the rounds I load on the Dillon they usually run the best next to 38 specials. I do have the undercut die for 40 and it definitely does the job. I have it mounted in my rockchucker and just sit there when I have the time going through 40 caliber rounds. I would say about 30 out of every hundred I reload need the undersize die to fit in the case gage. I do notice that these rounds that won't fit in my case gage will still easily drop right into my M&P barrel.