Yeah the double barrel is too weak and to compensate the ammo is plenty, you usually have 100. Its basically your pistol since there is no pistol in Quake. But there isn't really a viable shotgun replacement with that same close range power. Its probably the basic nail gun but there isn't that much ammo (the super nailgun is the machine gun).
You have to conserve nail gun for close range encounters against anything else than basic knights or marines, basically. The lightning gun is a Shambler and Vore killer but ammo is never plenty. Rocket launcher works on everything as well but has splash damage. The DLC kind of fixed this with more weapons (though some share the same ammo pool, but I liked the richochet rifle in Scourge), but I feel enemies are also more durable and faster there. The multi rocket launcher from the second pack is completely broken though, it annihilates everything.
I have yet to start with Dimensions of the Past, but its cool that this is brand new stuff.