If this gun was made before the 1950s, there is another option in addition to trying other varieties of factory ammunition...If you have a shotshell reloader, you may be able to fix it by loading rounds that have loose pellets in between old fashioned felt wads, instead of a modern plastic shot cup inside the shell. If it patterns properly with the old fashioned ammunition, then there is probably nothing wrong with the chokes, but rechoking could be necessary if you want to use commonly available ammunition.

Richard

ps, pigeon guns were used for competition, shooting live pigeons, which was done before greenies and clay pigeons.