This procedure will fix a loose receiver to barrel fitting on Mark pistols by taking up all the space worn by whatever between the square receiver lug hole and the frame lug it attaches to. My barrel became loose from over buffering during a re-bluing of the barrel. I did this procedure 4 years ago and the barrel is still as tight as I restored it. I field strip the gun to clean it after every use. Step 1. Break the gun down to receiver/barrel and frame. Step 2. With alcohol or acetone clean out the square hole on the barrel that the lug on the frame fits into, no oil based solvents please. Step 3. With a cotton swab place a film of oil on the lug in the frame… so nothing will stick to it. Step 4. Mix a small amount putty epoxy sticks like JB weld, the kind you can form and shape. JB weld has a high enough temp range well within the guns range. Step 5. Place a small amount of the mixed putty epoxy in lug hole on the barrel. Step 6. Reassemble the barrel to the frame and make sure you install the bolt stop completely in the assembled barrel receiver. You may have to bang the front of the barrel to get it far enough down to install the bolt stop. If you can’t reassemble the bolt stop you’ve put too much epoxy in the lug hole and the receiver is not going far enough down on the frame… remove excess and retry. Step7. After you’ve reassembled the gun, disassemble it again. Step 8. Now remove any residual putty epoxy from the barrel and frame that spilled out of the lug hole. You just want the epoxy to be in the lug hole on the barrel. Step 9. To let the epoxy fully cure leave the gun disassembled overnight before the final reassembling.