This represents an old trick they taught us in the US Cavalry. If you're sighting in a weapon but are having trouble getting a tight shot group, you can analyze your spread thusly.
-If your rounds are arrayed from left to right, you're most likely not pulling the trigger properly/uniformly; try squeezing the trigger gently, with the pad of your index finger. Don't anticipate the shot, just squeeze slowly and gently.
-If your rounds are arrayed up and down, you're not breathing properly; try firing each shot at the same point in your breathing cycle.
-If your rounds are unpredictable and follow no linear pattern, you're either guilty of both of these marksmanship errors, or you are failing to maintain the same sight picture; ensure that every time you look through your irons, you line up your front sight post and your rear aperture with the same orientation. You can look through the aperture any way you like, so long as you maintain a consistent sight picture. An easy way to get used to this is to stay focused through the aperture even in between shots. Once your sight picture discipline is strong in habit, you'll have no problem maintaining consistent shot groups all day long.