Before you even go to slap leather, think about how you are standing. No matter whether its target shooting, plinking, or defensive combat, remember that the first two are a simple evolution of the last. If a martial artist, be it a boxer, a foil fencer, an MMA competitor, or a wrestler stepped into the ring or arena unprepared and their balance off center, how are they going to be able to react to their opponent?
The same logic applies to shooting, be it with a rifle, handgun, or shotgun. If you have a sloppy method of bringing your firearm into action, you cannot blame it on the target.
Back in the old days, shooters loved the one-handed aim point stance for handguns. This rather delicate method bladed the body with the strong side pointed towards the target or threat, and the off-hand trailing behind the back. This is still how many Olympic style target shooters fire handguns.
The shooting stances of Mr. John A. Dietz, a well-known target shooter, Mr. E. E. Patridge, a police inspector, and Sergt. W. E. Petty of the Boston Police Department from a century ago. Drawn from the 1904 book, Revolver Shooting. This was the standard method of shooting for nearly a hundred years.
This old-school stance evolved in the late 19th century, drawn from the art of fencing. Think about it, in fencing and sword fighting, how does the competitor stand? You can take the above image, remove the guns, replace them with swords, and totally see it.
There are numerous problems with this stance, as we know today. For one, you have a wobbly grip due to only having one hand on the gun. Second, with your strong side foot forward, if you go to step or have to move, you will have to rotate your body awkwardly to "catch up" your off-hand side, losing a precious second or two, which can cost you points in a competition or your life in a gunfight.
A better and more appropriate stance is more like that of a boxer, or modern MMA fighter, both feet toward the target, head, face and hands oriented in the same manner as your feet.
Here we see a more modern stance and the geometry behind it. The shooter's elbows are positioned parallel to the ground with only enough bend to keep circulation. His back is angled towards the target. The eyes are lined up with the sights of the handgun that is held in a proper two-handed grip. Note that the pistol is brought to the eyes and not the head to the pistol. This is essential to proper shot placement.
This works well for either target or defensive shooting. It doesn't matter if you do weaver, modified weaver, isosceles or what have you, if the geometry isn't right, you are going to have a problem.
Further, should this shooter have to displace or move, he can effect a lateral change and still keep his weapon on target.
Any of the opposites of the above stance geometry will spoil a shot. For instance, if the elbows are slack and loose, the back leaned rearward, the arms overly locked or alternatively floppy, or the head scrunched down to see the gun, you are going to have issues on the target downrange.