Again, let me start with the story. The name comes from the name of sheriff Weaver, who had invented the stance in the 70's of last century and has been successfully applied in IPSC competitions which were already popular.
Weaver in the uniformed services has replaced the original version of the Isoceles stance and shooting with one hand, which was still popular in circles outside the FBI. The reason was that Weaver reduced the area of profile returned directly to the target while shooting. The idea was to reduce the chance of hitting an officer not wearing a bulletproof vest. Contemporary experiences shows that such an approach is not the best one. Nevertheless, this stance had important advantage - it was much more dynamic than the classical Isoceles and much more stable than shooting with one hand.
Basic principles of the Weaver stance:
- Feet astride roughly shoulder width apart
- Leg on the side of a hand holding a gun more or less withdrawn backwards
- Body twisted at an angle of about 45 degrees
- Support arm sustains a weapon - a bent elbow is lowered
- A hand holding a gun is also slightly bent with the elbow pointing down
Modified Weaver (also called by some Chapman's stance) looks essentially the same, except that the stronger hand is completely straight and elbow locked.
- Awkward arms position (especially when wearing tactical equipment)
- Highly isometric forces adversely affect the repeatability of shots during rapid shooting
- Exposing the less protected side of the body (when wearing bulletproof vest) / exposure to got shot through both lungs at once
- The limited range of movement / aiming to the weaker side
Below an instructor presents a modified Weaver stance and then an Isoceles one: