I am new to 3 gunning, but have already learned a lesson about shell caddies which pertains to the shells themselves. Not all 2 3/4" shells are sized the same!
The AP Customs give me the most round storage per inch of belt space, however, my Fiocchi slugs to not easily fit in them without binding. I tried a friend's carrier (not an AP Customs), and it was difficult to even try to load them. While I will continue to use a round dense carrier like AP for most of my rounds, I am planning on adding a carrier like the Carbon Arms Pinwheel that does not rely on the shell to be within a certain manufacturing tolerance to work properly. I'm thinking that even if I find slugs that I like, I could still run into problems down the road if I need to borrow shells from someone, or as is common right now, have to buy brand whatever because that is all I can find.