Items needed are: a Mark III pistol, a paper clip, as soft plastic hammer, and a needle nose plier or a small screwdriver.
If I am not mistaken, the procedure is the same for a Ruger Mark II pistol.
Step 1: Safety First
Remove the magazine. The magazine release button is under my left thumb. Hold it down and pull the magazine from the bottom of the pistol handle.
I am working on newsprint to protect the table surface and to protect the gun parts.
Step 2: Safety Continued
The Mark III has a thin wedge on the left side of the receiver that protrudes from the side of the gun if a bullet is in the chamber, but I prefer to look, anyway. A red dot is visible on the wedge when a bullet is in the chamber.
If the pistol cocks when you pull back the bolt, pull the trigger*, or you will not be able to remove a vital part in the next steps. If the bolt does not return to its closed position, find and press the bolt (or slide) release at the top of the left side handle. Then pull the trigger, if you have not already released it. You may need to insert an empty magazine to accomplish this. Remove the empty magazine when the pistol is no longer cocked.
*Note: While dry firing a firearm is very much discouraged, the Ruger's manual says it is no problem to dry fire this gun if the internal lock is disengaged, the safety is in the F or "off" position, and the firing pin stop has not been removed from the bolt. See the manual for further details on any of these items.
Step 3: Open the Mainspring Housing
Step 4: Open the Latch
Hook the round end of a paper clip over the mainspring latch and pull toward yourself. It pulls a little hard. Use the thin nose pliers to pull the paper clip.
Step 5: Latch Released
Step 6: Remove the Bolt Retainer
Step 7: Remove the Bolt
Step 8: Clearing the Way
The view in the photo is into the back of the pistol's handle. The yellow arrows identify the parts. Much of the hammer is in shadow. When the hammer moves, the hammer strut easily falls forward of the crosspin. Push the hammer in the direction of the red arrow. The tips of the needle nose pliers or a small screwdriver work well for this. Point the barrel of the gun upward so the hammer strut can fall toward the rear of the gun out from in front of the crosspin (as shown in the photo). Now the bolt should be free to slide out if it was not before.
Step 9: Remove the Barrel From the Pistol Frame
When the barrel releases from the frame, it will do so quickly and will fall. Grasp it between your knees or hold it low over the table to prevent damaging anything.
Step 10: Position the Hammer Before Installing the Barrel
The safety should be set to its lower position so the "F" shows in the opening. See the red arrow for the location of the safety.
Step 11: Pound the Barrel Onto the Frame
Step 12: Align Barrel and Frame
Step 13: Hole Alignment
The holes in the frame and the barrel for the bolt retainer pin must align or the bolt retainer pin will not go into place. The bright metal in the hole is from the pistol's bolt and this piece has some freedom of movement. It is not a problem. Hole alignment is the reason for the previous step.
Step 14: Insert the Bolt
The factory manual warns against damage to the firing pin if the bolt is installed without the firing pin stop. This is a pin that runs from one side of the bolt to the other. I have not yet disassembled the bolt. Unless you do, this will not be a problem to watch. The bolt will not go into the barrel upside down. There is only one way it fits.
If the bolt does not slide into the barrel, wiggle it a little. If anything obstructs it, check step 8. Turn the pistol handle up and see that the hammer and the hammer strut are not in the way.
Step 15: Install the Bolt Retainer Pin
Step 16: Finish the Process
Any problems I have had in the assembly of the pistol or in its operation have been due to the hammer or the hammer strut being out of their proper place.
Finally, wipe away any fingerprint oil you might have left on the gun.