You ask about how do I know about undershooting. For one, shooting at a stationary target, aiming at the center and most of the shot hitting way below center. For another, shooting pen raised birds for retriever training, I usually hit them low in the tail and leg area. And thirdly, on shooting skeet, which I don't do very much, I usually just nick the lower part of the rim, if I hit it at all. I can't remember when I dusted one. I have used a friend's Ruger 20 ga. o/u and my shooting improves. I would love to get an o/u, but money is a problem. Also, several shooters who are excellent at skeet have told me that I am undershooting.

Make sure you're not pulling your head up off the gun. That's what I think you're probably doing. Especially if it's slapping you in the face. If you had the gun tucked in properly and your cheek on the gun, that shouldn't happen.