I used 12 gauge and a Radio Shack relay
I can't seem to find a good place to mount my 12 gauge to the bike, even with the pistol grip and short barrel. On the handlebars compromises handling and atop the side bags gets in Brenda's way, not to mention the relay? Now, I really think a relay would not be necessary with a 12 gauge, unless you mean handing it off to the passenger so you could focus on driving while they do the blasting.
A heavy duty switch rated at 15 amps or above would be no problem if you don't use a relay
But if you use a relay, you are, in essence, getting a heavy duty switch rated at 30 amps. Sounds like a no-brainer.