As for capacity My 1100 can run with up to 23 rounds with my X-Rail installed.
The x rail is an interesting product for a serious open class shooter ( or a less serious shooter with the funds) on a dedicated open class shotgun. For an actual fighting gun or defensive weapon it is a notable draw back that one needs to run a 26" barrel to get 23 rounds.
I run a S12 with a magwell, if I am inclined to shoot open class (I'd consider my self far from a serious 3 gunner BTW).