What is the best and most preferred method for tightening up the bolt of a Winchester Model 12 shotgun?? It seems a little loose when pushing up on the follower when it's locked into battery. I have read where the top of the
inside of the receiver must be welded and then machined to alleviate this problem. Are there any other solutions that can be used other than the welding to fix this?? Thank you!

It is about a 90% chance it is not the top of the receiver that is the problem. I have been reconditioning Model 12's {among others} for 25+ years and have yet to see one that had a worn out bolt notch in the receiver. The part that wears and causes what you are describing is the action bar. You can get a new one from a company called Nu-Line for about $100.00 or so. They bought all the NOS parts to the Model 12 from Winchester some years ago. You can of course weld up the little pad on the end of your action bar and that will do it. Does the gun blow itself open when you fire it?? Not that I would wait for that to start happening, if the bolt moves up more than 1/8" it's time to fix it. Do not let some genius talk you into welding on the receiver or bolt.