About 90 percent of the rounds that I fire in practice are offhand or offhand with field expedient rest.
A good low cost drill I use often is to throw out a handful of spent 12 or 20 gs, shotshells and "keep then rolling" with the 10-022.
sounds ceasy, but..........
My thought is that if I can hit the target standing, then shooting from kneeling or sitting is a piece of cake..
Prone is very seldom practical in the hunting fields, so I use it very little. Besides, if I have time to get into a sling and go prone, I have time to stalk closer.