This is how I check my shotguns for fit, and stock length. It also will show if your p.o.a is hi or low, left or right to a certain point. I have a small flashlight I use to see how clean my barrel is on my 50 cal. caplocks. I can drop it down the barrel of my shot gun and pick a spot on the wall,( in a dark room ) and shoulder my gun at that spot and it will cast a 3 inch spot of light at 15 ft. I can also see where my bead is, in the light on the wall. I have a short l.o.p so most times when I do this with a stock that is to long, my barrel will be pointing to the left a little (right hand shooter) and when I shorten the stock, it will bring my p.o.a more in line. It's just a starting point, but it works for me. Jerry