I also adhere to the 83 degree pitch.However,the pitch controls how much the shotgun will or will not slip from your shoulder along with the felt recoil. Todays recoil pads nullify a lot of that.You find find the pitch on the old Model 94 Winchesters are a lot different than what you see on a shotgun
A side from a zero degree pitch, placing a shotgun against a wall is about as useless in determing pitch.Length of barrel and LOP will affect how much it stands out agaisnt the wall.It is about effcetive as the old wives tale of placing the butt of you gun in the crook of your elbow and see where your trgger finger is. When in fact, your eye needs to be about 1.5" in back of your thumb. There is no correllation between that and the length of your forearm.
Also, when patterning a shotgun,a person needs to shoot it as he does when hunting or shooting clays. Many of us do not consistently get a good cheek weld and a shotgun needs to be mounted, swung and pointed. As a professional ,this guy has it down pat, but most if they were to do as he does will find thier pattern is not were they think it is.
I think, O/U shoot differently than SXS, as the SXS will typically do a little dip and have a person shoot low.
I have a 1936 Ithaca that is giving me fits. Even after raising the drop at the heel 1/4 " ,it still shoots 4" low.
As stated you move the butt of your shotgun in the direction you want the pattern to go.Just like adjusting iron sights on a rifle.Same holds true with cast off and cast on to get the pattern shooting more center of left and right hand shooters,which no one seems to worry about.
