Weaver vs. Isoceles
Just so everybody is clear on the differences between the various shooting stances:
TRADITIONAL WEAVER STANCE
--Body bladed about 45 degrees in relation to the target
--Legs are locked at the knees
--Firing arm is slightly bent at the elbow (pre-lock)
--Support arm is sharply bent and the elbow points down at the lead foot
--Firing hand pushes out
--Support hand pulls in
--Because the bent arms may lower the position of the gun in relation to the head, the head may have to be tilted to the side to achieve proper sight alignment.
The advantage of the traditional Weaver Stance is that the bent arms and isometric tension of the pushing and pulling muscles create a shock absorber effect that reduces felt recoil and snaps the gun rapidly back on target. Since the gun is closer to the body, it feels lightler and in fact exerts less leverage weight on bent arms than it would on fully extended, locked out arms.
The disadvantages are that the Weaver Stance is uncomfortable for many people. Shooters with shorter arms, greater upper body mass, or women with big bosoms (!) find it difficult to blade in relation to the target and reach across their chest. Sometimes the strong arm will over-power the weak arm, sending bullets hight to the left side for the right handed shooter. More often, the shooter may not lock the elbow of the support arm down enough, which results in the stance becoming unlocked and causes shots to drift low right for the right handed shooter.
THE CHAPMAN MODIFIED WEAVER STANCE
--Body slightly bladed in relation to the target
--Weak side foot forward
--Strong side foot back
--Weight balanced slightly on the lead foot
--Center of gravity slightly forward
--The foot position should be like driving a punch -- the forward leg bears the weight and the rear leg is the drive leg
--Elbow of the strong side arm is locked or nearly locked (pre-lock)
--Elbow of the weak side arm is bent down and aimed at the lead foot
--Lead shoulder over the lead knee
--Isometric Tension -- strong hand pushes out and the weak hand pulls back
--Bring the head down to the sights. If you bring the gun up to your eye, you may shoot high
--Cheekweld the side of your jaw on the strong side upper arm, just like cheeking a rifle stock. This consistently positions your eyes in relation to the sights, the same way every time
--Wide stance/pyramidal base
(this stance is my personal preference. I find that bringing my head down to the sights, establishing a cheek weld with the upper arm on my strong side, and keeping the center of gravity forward works very well when firing multiple rapid shots or when engaging multiple targets.)
THE MODIFIED ISOCELES STANCE (aka "the turret")
--Wide stance/pyramidal base
--Weak side foot slightly forward
--Strong side foot slightly back
--Weight on the lead foot/shoulders forward of the feet
--Center of gravity slightly forward
--Arms locked out
--Slight crouch/knees unlocked
--Lean into the gun
With the torso bent slightly at the waist and the gun straight out ahead in both hands, the body is balanced by the flexed knees, which automatically compensate for balance by lowering the center of gravity for the body in the pelvis. This technique can be made even stronger by taking a step toward the target with the weak foot and bending the lead knee, applying the weight forward. Think of it as leaning into the gun.
The body is now poised to move instantly forward or bak, or side to side, and a considerable portion of upper body weight, coupled with the muscular tension of the locked arms, helps snap the gun down in recoil.
The modified Isoceles Stance is essentially Isoceles from the waist up and Weaver from the waist down, and seems to be the most comfortable for many people.
I find that most shooters find the modified Isoceles Stance the most comfortable, followed by the Chapman Stance (in about a 60/40 ratio). I find almost nobody using the traditional Weaver Stance anymore, unless they're an old time IPSC shooter. In my experience, Shooters with longer arms and more upper body flexibility generally tend to find some kind of technique that looks like a Chapman, and shooters with shorter arms or greater mass in their upper body prefer some variation of the Isoceles.
(I personally prefer the Chapman Stance)
It's all a matter of personal preference, and preferences and comfort levels will change. I know a couple of IPSC & IDPA shooters who had back or shoulder injuries that made them switch from a modified Weaver to an Isocles. We have one guy on the PD who shot from Weaver who tore a rotator cuff making an arrest. When he came back to work after the surgery, we had him experiment with Isoceles to see if that might work better for him, and it did not. In his particular case, Isoceles was actually painful and the modified Weaver worked much better . . . I know another guy (an IPSC & IDPA shooter) who shot using the Chapman Stance for YEARS and then when he got old and had to get bifocals, he found he had to switch to Isoceles to get the front sight out just a little farther so that it was back in focus with his new glasses.
There is no one best way, there are just alternatives, and you have to experiment a little bit to see what works best for you. If you're an instructor, you need to know and be able to explain and demonstrate and trouble shoot all the common variations, because you don't know what combination of techniques will work best for your students.