You can learn to shoot: running, walking, as you sit down, as you rise up, in low light, while covering 360 degrees. Except for hand placement on the weapon, the human body is ergonomically designed for the same technique for shooting pistol, rifle, shotgun and submachine gun. It is a matter of framing, indexing and cuting the target in half. turnipseedtechnique.com and quickshoot.com.
The inability of a person to pick up a firearm weighing between three and twelve pounds and point it at a target is an entirely different animal.
In two separate classes I witnessed the same thing. These were people who never touched a firearm in their lives. With just a few hours of hands on training, overweight, middle aged housewives who last exercised in high school were able to do the following. Stand 12 feet from a target with an empty Glock (for you Glock haters, use a Government Model) in hand. On the ground is a piece of cardboard and on top is a loaded magazine. The shooter is to pick up the magazine and engage the target as fast as the trigger can be pulled. When the gun is empty, take out the magazine, put the gun on the ground. Then run 50 feet to a picnic table and throw down the empty magazine, pick up a full magazine, run back and pick up the gun and engage. Do it until 50 rounds are expended. Despite wheezing, sweating, shaking, the women did all kill shots rapid fire. So, weight training has nothing to do with it.
If you get the turnipseed video (and no, you will not learn how to shoot by watching a video), he has a trained shooter who had never touched a machine gun, pick up an Mp5 and rapidly advance on a target firing full auto and accurately. It is a middle aged, overweight female, of course.
With parents' permission, I took a slightly built 11 year and taught him the operation of a Glock and correct framing sitting and standing and let him go - 1000 rounds in two days. Yes, he did get fatigued after a few hours each day.
If you want to shoot old fashioned bulls eye style with one arm locked and the hand of the other arm in your pocket, no amount of weight training will make up for this bad posture. If you insist upon shoot a 12 ga. shotgun with slugs for two days using a modified weaver or an isosceles position, you are going to end up with bruised or bloody shoulders and no amount of weight training will help.