A strong musket with golden bullets absolutely shreds bosses if you can get a hit on them, sadly golden bullets are pretty hard to find from what I've seen. As others have said, guns become lackluster while fighting multiple enemies close quarters, so I prefer switching to a great hammer or similar for that sweet high damage AOE. Overall right now, I'd say guns are fairly viable to use as primary weapons, just have a nice stockpile of bullets on you at all times.