The Migratory Bird Act of 1937 put an end to waterfowl hunting with any gauge larger than 10 gauge. That's FedReg. Most states aligned their resident game laws to comport with that standard soon afterward. On CA State waterfowl areas, 12 gauge is as large as you can go, and the 3-1/2" Magnum with non-toxic shot is the standard for whacking quacks.
I have no experience in shooting an 8 gauge shotgun, and no interest in acquiring same. I do have a 10 gauge shotgun, and it has gone along on turkey hunts with me in years past before the coyotes killed off all of the local wild birds. That old 10 bore might get a trip or two this Spring, as I have located a decent flock or two not real far away. I love getting "gobble-backs" to my calling.