The shotgun I find to be useless is the one you get from the Hodunks after the Gang Wars quest, I have tried it in several different ways and can't find one useful scenario for it.
That stupid mine-launcher thingie? (The Triquetra is pretty cool imo) ... i think it can regen rocket ammo in conjunction with the sham ... but i prefer a Logans gun or TBF for that purpose
That sounds like it, it will shoot out four mines in a square pattern and then they will detonate after about 10 seconds if not more. With as fast as most enemies are in this game it just takes too damn long to be of any use against them.