I've had a Dillon 450 for will over 30 years. Even looking at their newest catalog that I got last week, I can't see any difference between it and a 550, other than some available add ons for the 550. My Dillon will accept any 7/8" dies. I use RCBS, Lee, Hornady (it doesn't matter) dies in mine.
I mainly use my Dillon, with their powder measurer, for all of my pistol reloading and my .223 and .308 Win reloading. I use carbide size dies for pistol reloading and Lee collet neck size dies for my rifles. My .223 and .308 rifle reloads are consistantly accurate enough to hit golf balls at 100 yards, a 10" steel plate at 300 yards, and the 15" steel at 430 yards.
I shimmed up under the die holder on my Rockchucker press so I can interchange my loading dies between my Dillon and Rockchecker presses without having to make any adjustments.
