I have done it at a gun show without removing the stock. Removing the trigger mechanism and pushing the breech bolt forward and holding it in the full "up" position while you assemble the gun and get the slide into position solves all problems. Then you can reinstall the trigger mechanism. It requires two very talented hands to accomplish, or preferably, three hands.

The "Jack The Dog" model 12 set may be a good shooter, but no collector. Refinished with non factory checking and pad plus the mentioned add on barrel assembly takes it totally out of collector class.

This thread got me thinking about all my model 12s and 1897s. The first one I ever bought was a pretty well used 12 gauge that I purchased from a gun shop in Worcester, Mass. My wife and I were on my first "big" motorcycle, a Honda 750 4. We lived in Sturbridge, Mass. At the time, about 20 miles from the gun shop. We drove home with Sylvia holding the gun over her shoulder. I remember passing a State cop and he never even slowed. Try that today.

I thought those were all full impressive motorcycles in their day.

They were and a friend had one with a 900 cc kit and a turbo charger. He let me take it for a spin. He told me to take it easy as it was pretty hot. Not being too smart, I went through the gears at the red line and when I dropped it into 5th, at well over 100 mph, the front end came about 4 feet off the ground. When I got everything back under control, I took it back to my friend at about 45 mph. Mine was fairly stock with only a cam and pipes, so I did not really know what to expect. Now, I do.

Model 12
 

Remove trigger group, push bolt forward and up and hold in place. With small screwdriver engage the breech bolt lever lock( seesaw looking lever on side of bolt. ) this will hold bolt in place ( this normally is engaged when action slide is moved forward during disassembly) while you reassemble trigger group. Take care that the cartridge cut off is properly fitted into it's slot, don't force anything. When front end is reinstalled and action slide engauges bolt it will unlock the lever.

Good luck.

Sat down in my patio with my 16 gauge model 12. Had my set of gunsmithing screw drivers. Fixed it in about three minutes. I've been inside that beast enough times so that I can pretty well keep myself out of trouble. A big secret is to push the bolt down (when the gun is upsidedown) when you insert the operating rod handle. The gun goes together easily when you do it right. Do it wrong and you are pretty well screwed.

Glad it worked out easily, sometimes the simplest things can be the secret.
Go shoot the heck out of it.