Gentlemen, I would appreciate suggestions for managing and fireing a too big for offhand, long, heavy, scoped pistol for good deer hunting accuracy under field conditions.

gwahir, I use those, what you describe as, long, heavy, scoped handguns for deer hunting. Depends on your method of hunting: I hunt from elevated blinds and use a sandbag off the window ledge. Very solid!
I welded arm rest on home-made ladder stands where I could rest my elbow along with using one of my knees to steady the handgun. This worked well when I was hunting from a ladder stand.
If I found myself sitting by a tree, I would rest the gun off both of my knees placed together.
Open country deer hunting may allow for a backpack to be used as a rest. I also have a home-made "snipers cradle" on a camera tripod that's kinda neat.
Just a few ideas I've employed in the past.
Best of luck with whatever you find that works best for your style of hunting.

Thanks, Mark. This 'pistol' is actually over 26" long, so I could make a detachable buttstock. Works well off the bench the way it is, but need more stability if sitting with my back against a tree.