Laser bore sight tools are relatively inexpensive. With such a device, you can verify that your sights are aligned with the bore. If that is the case, you would expect your bullets to make a tight pattern around your point of aim. That doesn't always happen.
There is a very long list of variables that influence where a bullet actually goes. I will only touch on a few of them here. It can take a great deal of practice to keep the required act of pulling the trigger from mis-aligning the sights. I have heard more than one training instructor tell right handed students involved in an altercation to shoot the assailant's right pants pocket. Left handed shooters are told to shoot the assailant's left pants pocket. “Squeezing” the trigger under high stress causes right handers to pull the gun up and to the right. Lefties pull the gun left and up.
A series of things happen as the trigger is pulled continuing until the bullet departs the barrel. While it may not be a big deal, there are parts moving in the gun. They may be small, but they still generate forces. More important to our discussion are the reactionary forces created by the expanding gasses and bullet leaving the barrel. The forces of recoil start to influence the gun and sight alignment before the bullet has left the barrel, meaning the barrel will not be pointed where it was at the time of primer impact and powder ignition.
How can reloading help this problem? Heavier bullets require a higher or taller front sight or a lower rear sight. Heavier bullets tend to shoot higher because the forces of recoil are higher with heavier bullets. Shoot lighter bullets and the short range point of impact will be lower.
Conversely, if your point of impact is below the line of sight, then shoot heavier bullets, increasing recoil and raising the point of impact.
Experienced shooters know that holding a handgun the same every shot is a vital part of accuracy. Holding the gun with the same pressure and position every time causes recoil to effect the gun the same way each time. Every shooter is different. A gun with sights regulated for you with a specific load may not regulate well for someone else.
Obviously, this will only work to a certain point. If lighter or heavier bullets still don't solve your problem, it is time to change your sights. On many guns, this is easy to do and doesn't require a gun smith. Order different height front and or rear sights from the manufacturer and replace them. If this isn't possible, there are still remedies.
If your gun is shooting low, note how far down the front sight the actual point of impact is. It is easy for a gun smith to file the front sight to more accurately align the sights with the point of impact. It is easier to remove material than it is to add metal to a front sight so take it slow.
There is an interesting formula to help you determine “how much.” * Sight Radius (inches and 10ths) X Distance to move group (inches) / Distance in inches to the target. For example, sight radius of 5” from back of rear sight to back of front sight. Gun shoots 3 inches low at 20 yards. 20 yards =720” Our formula looks like this. 5” X 3” /720” = 0.02”. The sight needs to be adjusted .02” which isn't very much.
If your gun is shooting right or left, “crankin the barrel” is possible, but I would leave this task to a qualified gun smith with a very good barrel vise.
I hope that finding the right load is all you need to do to “regulate” your fixed sight handgun to actual point of impact.
Have fun reloading and shooting.
*Guns Magazine, March 2011, Vol. 57, Number 3, 664 Issue, Q & A, Jeff Johns page 42.