I have a ruger NM blackhawk, the hammer won't move all the way down to rotate the cylinder. When the cylinder is not in the revolver I am able to move the hammer down freely, but when I put the cylinder in the hammer doesn't move.
When you close the loading gate do you align a cylinder chamber with the bore and get the cylinder to lock into place before trying to cock the gun?
This puts the cylinder "in time" with the hammer and pawl mechanism.
This is an important step to loading and firing this gun, as noted in the NM Blackhawk owners manual.
It also sounds like the rear frame is making contact with the cylinder. if you can hold the gun up to the light and see if you have clearance between the frame and the cylinder. i have seen where they have been macheined a little off on some of the new modles or the cylinder is making contact to tight with the forsing cone. hay you never know.....