3) I said held in one had casually, one it's not casual when firing a weapon. I was refering to when someone is just HOLDING it when not being engaged.
Actually, it's recommended to use only 10% of full muscle tension when operating a firearm, in order to eliminate the shaking due to your muscles steadying the weapon (
Marksmanship by Gary L. Anderson, copyright 1985). It's all about using the bone structure to steady the weapon, while the muscles do as little work as possible.
With handguns, using your muscles over your bone structure is pretty much a requirement. However, with submachine guns and PDW's, you can apply the same tricks as used with rifles. Mainly, the use of a tactical sling around the back of the left arm (above the elbow) will serve to support the weight of the weapon, while the positioning of your left hand adjusts the elevation of the weapon.
You could also jury-rig a sling similar to the "whip-it" gun (sawwed-off shotgun). Originally, this weapon was intended to be slung from the right shoulder with a small loop of leather. This allows the user to bring the shotgun to shoulder level in no time (hence the term "whip-it", as it whip it out), while keeping the weapon hidden underneath a coat. This eliminates the need for a tactical sling, scabbard, or holster, while giving you the bone support required to accurately aim the weapon.