Overtravel Adjustment of the Ruger Mark II Target Model
If your MKII came from the factory with adjustable sights, then it's a target model, and it has an overtravel adjustable trigger. Overtravel is any distance that the trigger moves after the fall of the hammer, and it is probably the most crucial aspect of trigger pull in a target gun. The reason is that you can learn to overcome a creepy trigger, or a heavy trigger, but once the hammer falls, any motion of the gun will move it from the point of aim, and no one's reflexes are fast enough to stop moving as soon as the tension on the trigger is released. The overtravel screw is a royal pain to get too, so expect to spend a couple of hours doing this. Learn how to disassemble and reassemble the gun, including taking the receiver tube off the frame. To remove the trigger, you have to take out everything but the sear and magazine release. Read these instructions several times before you start, and look at the exploded parts diagram; it will help.
MAKE SURE THE GUN IS UNLOADED AND THERE IS NO AMMO ANYWHERE NEAR YOU WHEN YOU DO THIS!!!! THIS PROCESS REQUIRES DRY FIRING, SO ALWAYS KEEP THE GUN POINTED IN A SAFE DIRECTION!!!!
Location of the overtravel adjustment screw.
Once you have the trigger out, clean the frame in front of the trigger very well, then take a small allen wrench and unscrew the set screw on the front of the trigger 1 full turn, reassemble the gun, and see if pulling the trigger drops the hammer. If it doesn't, then unscrew it a turn at a time until it won't fire any more. You now have a negative overtravel, and we'll slowly search to find the exact threshold where the hammer falls. Once the hammer stops falling, screw the screw in 1/2 turn, assemble, and test. If it still fails to fire, screw in 1/4 turn; if it does fire, screw out 1/4 turn. Repeat the process with 1/8 and 1/16 turns, in if it fails to fire, out if it does fire (any programmers out there will recognize the binary search algorithm here :). At the 1/16 turn level, the overtravel will be down to less than 1/100 of an inch. Once you get it as small as you'd like, apply a drop of LockTite or SuperGlue to keep it from moving, then try once more to make sure that it didn't back out any. Now take it out to the range and see how much better you group.
NOTE: Since there is no overtravel left, you'll need to keep the area in front of the trigger clean; any buildup of dirt and the trigger won't move back far enough to release the hammer, and the gun will stop working until you take it down and clean it.