You can tighten the receiver/frame fit on the MK I, MK II and MKIII Rugers to get rid of the looseness using a good vise and and a little patience. Using a vise to tighten frame/slides/receivers etc. in a variety of guns is a time honored 'smithing technique Here is how to do it on the Ruger MK's.
1. Remove the barrel/receiver from the frame. You do not have to remove any of the internal components in the frame (trigger, disconnector , hammer etc.)
2. Pad the jaws of a good vise with brass or lead so you don't mark the frame.
3. Place the frame in the vise, pointing down as if you were shooting the floor, with the jaw surfaces parallel to the side of the frame.
4. Position the jaws so they will squeeze the flared flanges at the top of the frame in the area between the trigger pivot pin and the front of the trigger guard. The trigger pin should be visible above the jaws of the vise. What we are trying to do is reduce the diameter of the curved area that the receiver sits in. This will cause the receiver to sit higher in relation to the latch boss in the frame which causes the two surfaces to mate more perfectly.
5. Snug up the vise and with a magic marker make an index mark on the vise body and vise handle so that you know where you started.
6. Turn the handle on the vise about 15-20 degrees. (You don't need a protractor to measure the angles. If you envision a 45 degree angle and divide that in thirds you'll have a "close enough" estimation of 15-20 degrees.) Make a new index mark, remove the frame from the vise and try putting the barrel/receiver back on. You should have to lightly drive the receiver/barrel onto the frame with a wooden or leather mallet when the proper fit is achieved.
7. If the receiver /frame fit is still loose (doesn't require force to put it on and off the frame) put the frame back in the vise, tighten another 15-20 degrees from the last index mark, make a new index mark, and try the fit again. Repeat step 7 until you achieve the desired fit.
CAUTION
1.Go slowly - if you over tighten you won't be able to get the receiver on the frame. Go only as tight as you need to get rid of the looseness.
2. DO NOT squeeze any area other than that which is described. !!!! The front of the trigger guard and the latch boss maintain the internal frame dimensions allowing only the lips/flanges that the receiver sits on to compress. If you squeeze anywhere else the whole sheet metal frame will compress and cause all sorts of dimensional clearance problems for your internal working parts!!!
