If you are hitting "all over the place," the problem is not with sighting in. Something else is amiss, whether it be you, the gun, or the ammo.
Step 1: look for loose screws, parts, or pieces.
Step 2: buy 2 other brands of ammo. Just one box each. Try in particular to get different weights of bullets.
Step 3: shoot from a benchrest
The target type is irrelevant, although I prefer large pieces of paper (tabloid size) so I can at least tell when I'm on the paper. With small targets on well-used cardboard, you may have no idea where you are hitting.
Take your time, do everything by the numbers, and shoot a 5-round group. At 50 yards, from the bench, even with open sights, there's no reason you shouldn't have a group that you can cover with the palm of your hand. Repeat the process with a different brand of ammo and a new, clean target.
If you're getting a buckshot pattern, we're back to the "something is wrong" premise. Eliminate variables one by one. Start with what's easy, get rid of yourself. Ask someone else whose targets look a lot better than yours to give it a try.