With the receiver of the gun touching the wall, and the gun butt flat on the floor, you will observe one of 3 things in regards to pitch.
1. If the gun barrel or rib (other than a tapered rib, from chamber to muzzle) is flat against the wall, that gun is said to have no (0) or neutral pitch.
2. If the barrel of the gun tips away from the wall, that gun has positive/down pitch.
3. If the barrel touches the wall and the receiver does not, that gun has negative/up pitch.
Dave Berlet
https://www.youtube.com/watch?v=poEBXc32TRM Allen McCannon
https://www.youtube.com/watch?v=iF6bBg5rx3M Roger Pace
https://www.youtube.com/watch?v=71LRVQCRWeI Neil Winston - The effect of pitch on muzzle rise
http://www.trapshooters.com/threads/the-...-powers.328905/ 
