Looks like new shotguns are spawning in-game. The Tweet says they're in 'specific' areas but the community announcement says nothing about that. So hopefully the gearbox forums will help narrow the search grid.
Gear up for the release of Sir Hammerlock’s Big Game Hunt -- the third campaign add-on for Borderlands 2, arriving January 15 -- with special in-game events this weekend!
The festivities begin today, January 11, with Vault Hunters reporting an influx of powerful shotguns throughout Pandora. To join in the fun, simply boot up Borderlands 2 while connected to the internet and start looting before the supply runs dry -- based on current conditions, expect the excess to be exhausted on Saturday, January 12 at 10:55AM CST / 4:55PM GMT (What time is that for me?).