A 550 is a 4 station, manual indexed progressive. Progressive meaning that it's doing multiple actions at once. The toolheads have 4 stations on a 550 and so each stroke of the ram can do 4 things to different cartridges.
However, there are times where you may not want to run 4 things at the same time. For those situations there is a guy in Alaska that made a special spindle/toolhead combo that converts it from progressive (4 actions) to a single stage (1 action). Takes about 3 minutes to swap between modes.
For the extra 90ish bucks that the conversion costs you can have all the benefits of single stage for some rounds, as well as the time/bulk capablity of a progressive. Like, I'm not making 9mm in single stage mode, and I'm not making 375H&H in progressive.
A 650 (also a fine machine) is an automatic index progressive meaning it's basically the 550 but it automatically advances the cartridges around the stations for you. This can be a good and bad thing because it speeds things up alot --- until you have any malfunction.
However, there are times where you may not want to run 4 things at the same time. For those situations there is a guy in Alaska that made a special spindle/toolhead combo that converts it from progressive (4 actions) to a single stage (1 action). Takes about 3 minutes to swap between modes.
For the extra 90ish bucks that the conversion costs you can have all the benefits of single stage for some rounds, as well as the time/bulk capablity of a progressive. Like, I'm not making 9mm in single stage mode, and I'm not making 375H&H in progressive.
A 650 (also a fine machine) is an automatic index progressive meaning it's basically the 550 but it automatically advances the cartridges around the stations for you. This can be a good and bad thing because it speeds things up alot --- until you have any malfunction.