"How does the action, accuracy compare to the MK II, III series? These are hammer fired?"
The MK and 22/45 pistols have an "internal hammer" and are "single action semi-auto" while the SR22 has an "exposed hammer" and is "SA/DA semi-auto". The SR single action trigger is sort of OK but nothing to compare with the MK & 22/45.
Accuracy is a factor of design and the SR isn't what I'd call "designed for accuracy". Here's a personal comparison I can make:I have a 4" 22/45 that is capable of 3-4" groups @ 100 yards with the right accessories (3x scope, muzzle extension, sandbag rest)to test it's mechanical accuracy. The absolute best group I've seen from an SR22 is about 3-4" @ 50' due to the short sight radius, small grips, iffy trigger, and light weight.