I have a Sterlingworth 16 which is chamber sleeved down to 20 ga. The 20 ga wad easily seal the 16 ga. bore and delivers very good patterns. The chambers were trashed due to two paper shells being left in the gun for several decades. The chambers had pits that were like craters on the back side of the moon. Forced to chamber sleeve it I decided to go down to 20 ga and redo the extractor to make it into the long range dove gun of my dreams. The 30" mod and full barrels are great for long range doves. My son used it a few years ago on opening day and quickly bagged his limit but opening day doves are often fairly easy. He did make a few long range shots that were very impressive.
A few years ago there was a seller/dealer taking old 10 ga doubles and chamber sleeving them down to 12 ga and then selling them as pigeon guns.