By god those are some atrocious firing stances! Little to no weight on the front leg in any of them. Those characters would most certainly fall down when firing those guns!
challenge accepted:
http://www.globalsecurity.org/military/library/policy/army/fm/3-22-9/image1670.jpg
7-4. STANDING FIRING POSITION
To assume the standing firing position, the soldier faces his target, executes a facing movement to his firing side, and spreads his feet a comfortable distance apart (Figure 7-4). With his firing hand on the pistol grip and his nonfiring hand on either the upper handguard or the bottom of the magazine, the soldier places the butt of the rifle in the pocket formed by his firing shoulder so the sights are level with his eyes. The weight of the rifle is supported by the firing shoulder pocket and nonfiring hand.
The soldier shifts his feet until he is aiming naturally at the target and his weight is evenly distributed on both feet. The standing position provides the least stability but could be needed for observing the target area since it can be assumed quickly while moving. Support for any portion of the body or rifle improves stability. More stability can be obtained by adjusting the ammunition pouch to support the nonfiring elbow, allowing the rifle magazine to rest in the nonfiring hand.
http://www.globalsecurity.org/military/library/policy/army/fm/3-22-9/c07.htm
relatively unstable but not completely unrealistic. AND it looks better than vanilla (especially idle stances and walking animation).