The single disc set-up is "GE, good enough" as my father would say for anyone who isn't from Colorado. The graudations between cavities are close enough togeather to get you close enough to do just about anything. Any finer graduation, by using the double disc kit, is splitting hairs as far as I'm concerned.
I have the DD kit because I lost one of the four discs, and it was the cheapest way to replace it at that time (years ago). I never actually used the DD set-up, so I can't attest to it's reliability. I have never read anything bad about it.
I have the adjustable "disc" and only tried it once. I wanted small doses of Bullseye and it was an epic fail. It through wildly different weights. In fairness, it specifically says that it won't throw accurate charges with flake powder on small settings. The adjustability of it is lost on me. I like the LACK of adjustability of the discs. 0.37cc is ALWAYS 0.37cc. I don't need to check or calibrate anything if I want 3.5 Bullseye, just pop in the 0.37cc cavity, verify that it IS the 0.37cc cavity, and proceed to load.
I got the adjustable cavity free with the kit to upgrade my measure to the Pro or Deluxe or whatever they call it. This is definately worth it. It makes it much easier, neater, and faster to change discs or powder and the brass screws hold up very well. The standard hopper screws never gave me any trouble, but the hopper does seem a bit fragile. Anyway, with the upgrade, you can change powder or discs without remiving the body of the measure from the press.
ETA: One use for the DD kit is for large charges. 23 or so grains of 296 is about max with one disc, so if your loading 500SW you might need to double up
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Last edited by WiskyT; 03-14-2011 at 17:15..