I've been using a C-More Railway with my Black Mamba with a 12 minute dot for many years and have been doing pretty well with it. The dot is LARGE but I can still see the entire target. It's good for Steel Challenge but when I want to shoot groups, not so much.
I made a trade the other day for a 6 MOA dot and gave the other guy my 12. Today I got to sight it in. Even though the dot is in the same place a re-sighting is needed.
My elevation was off by 3 inches at 15 yards. Not much but I prefer center hits. Windage was also slightly off to the right. Took about 1/4 turn on the adjustment screw and started hitting center on the 6 inch sight in target.
Since Steel Challenge has plates 10, 12 and 18 X 24 dead on is not important. Basically if you are anywhere near the plate when the shot goes off you should be able to hit it. This hasn't been the case for me the past few years after coming back to shooting.
The 6 MOA dot really helps me center the scope and I think my times will be better. I'll find out in 2 weeks when we have a match.