I've noticed the same thing.It just seems obvious, given the mechanical differences that the Mark series pistols would provide superior accuracy over the Single-Six revolver...Where the post convertible Single-Six shines accuracy wise, is with the 22 Magnum cylinder installed. Some will argue until they're blue in the face about this, but every last one I've tested has provided far superior accuracy in 22 Mag, due to the bore actually being rifled for that particular cartridge.
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I do have a variety of single sixes. There are two in particular that I believe might come close to being as accurate as a Mark II.
Now! The next question is, which Mark series pistol is the most accurate? I've got a Mark IV 22/45 Lite that isn't as accurate as a Mark II. Far from it in fact.