The Flak Grenade is a standard secondary weapon for the shotgun.
The grenade flies along an arc and will shatter on impact into deadly flak. It does slightly more damage than the impact grenade for the machine gun, but the metal fragments can end up bouncing quite far, so using it in corridors can give you a few scratches.
The shotgun has some nice secondary weapons available, but unless you're using the absurdly explosive slaughterfaust you'll probably find the default flak grenade is worth sticking with. It's highly effective against both tough baddies at medium range and groups, both situations that the standard shotgun isn't well equipped to handle.