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I guess most if not all the contents of this post are already ninja'd but I'll post it anyway =)
I'm using rocket launcher only to destroy the wall. Then maybe a couple rockets to Barons (if I spam rockets I end up destroying goodies). I think it's safer to use rockets just for opening, then hide behind the corner and corner shoot with shotgun (and then chaingun, if you have the build).
This. I learned not to rely on the launcher after destroying the precious backpack a few times. I thought that one missed rocket is still safe, but the barons tend to open the doors leading to the goodies and somehow my rockets always manage to squeeze there and wreak havoc with the reward.
Another tip is to make a small alcove using a single rocket a few tiles below the hole (or above, depending on where you make it). That way you have somewhere to retreat and continue cornershooting if the knights/barons get dangerously close.
To answer your post:
I used to play a lot of AoHu games where you get one of each mod from start, so I could always mod the combat shotty if I found it. I tend to forget that isn't the case in every game. The combat shotty should be enough, though.
The DS does little damage at greater ranges, that is why you should wait until the enemies get close and then shoot it. It's got a lot of knockback though which should give you enough time to reload and do it again. So shoot with the combat shotty while they are far and once they close, switch to double and knock them back. By that time they should have only little health left and should fall to another almsot point-blank DS blast, I think.
Shell box (and other such items, such as ammo chains) indeed help with reloading - if you have it equipped in your prepared slot, reloading will take only 1/5th of the time it would normally take.