No doubt the older english doubles are found in a wide variety of calibers. The nature of the question about what is more common seems very appropriate for that portion of the guns in use. That said, I think you might find a different answer if you polled the modern day manuafactuers of doubles to see what is the most common caliber ordered by their customers. Whatever the answer is to the question about classic doubles, might the ultimate answer across the board be changing as the newer guns being manufactured possibly go in a different direction?