A lot of people say that shooting stance is a matter of personal preference, which is true, but at the same time there are certainly reasons why one stance might be objectively better than another, especially in specific contexts.
Thesis: If your goal is to shoot rapidly with minimal muzzle rise, then the ideal stance has your skeleton aligned to resist recoil, without needing to use a lot of muscle.
Recoil is something pushing you straight backwards away from the target. What would you do naturally if someone walked up to you and was about to shove you back, hard? You'd probably do something like a wrestler's stance. You'd lower your level by bending your knees, and square up to the person with your feet, knees, hips, shoulders, and head all facing them, and raise both arms up to about shoulder level with your hands facing them. You'd generally want one leg back so that your rear leg can give you a forward push.
Something like this:
This stance is ideal to resist a person pushing you, and it's similarly good to resist the effects of recoil.
That's because it has good skeletal alignment towards a target. Note: I'm not saying you would bend forward that much when shooting or otherwise look exactly like that; it's just to give you a general idea.
You are skeletally aligned if:
Your head, shoulders, hips, knees, and feet are all connected more or less in a straight line. You might be bent forward a little bit, but there should be no misalignment sideways.
In addition, your feet, knees, hips, shoulders, and head should all be facing in the same direction, with bilateral symmetry. In other words, your body is not twisted or tilted in any way. Like this:
You are misaligned if these 2 things are not true.
When you have good skeletal alignment, resisting something pushing you back is relatively easy and you can stay relaxed. When you are misaligned, you can still do it, but it takes a lot more energy and requires you to get more tense.
Let's say you had to do a bench press. Would you keep your head, shoulders, hips, and knees facing up towards the ceiling, or would you turn your knees facing to the right and your head facing to the left? Now, maybe you could do the bench press while misaligned, but you'd have to use a lot more muscle, and get tired faster. And probably injure yourself if using any heavy weight. Why make it hard for yourself?
In wrestling or grappling, your goal is to always keep yourself aligned and to misalign your opponent. In gunfighting, your opponent probably is not going to be able to touch you, so the only reason you'd be misaligned is if you did it to yourself.
I'm not saying everything has to be perfectly aligned and bilateral. For example, you probably would want to have one foot back, which gives you a base to the rear so you can push forward, which is not bilateral. And your rear foot might naturally feel more comfortable pointed out 10 or 20 degrees. A little bit of slack is fine as long as there is a good reason for it, but the general principle of alignment holds true.
Quiz: which is more aligned? Which would be superior for shooting quickly while minimizing muzzle rise?
Aside from the obvious twisting of your body when doing a bladed stance, also look at your shoulders. Often when you do a bladed stance, you end up with your shooting shoulder higher than your other shoulder, which is no good.
With isosceles your skeletal structure helps reduce the effects of recoil, so there's no need for the push-pull that Weaver requires. Having to do an isometric push-pull will cause your muscles to get tired prematurely, and is antithetical to being relaxed, which is required for optimum performance in any sport.
When using a rifle or shotgun, if you stand more square, you need a shorter stock. You should mount the stock a little more inboard towards your chest as opposed to out towards your arm, because you want to minimize the effect of having your shooting shoulder pushed back by recoil (which would reduce your bilateral symmetry).
When shooting any kind of gun, keeping your elbows pointed out sideways rather than down helps reduce muzzle rise, because when your elbows are pointing down, that naturally allows your arms to bend upwards (like doing a bicep curl). Some uber-tactical guys might complain that keeping your elbow out sideways makes it more likely to get shot off, and that may be true, but I wouldn't worry about it that much. You can always keep your elbow down when rolling out from behind a barricade.
When you mount a long gun you should always keep your head upright in a natural position without moving it, and raise the gun to your face, don't put your head down on the gun. Moving your head will usually end up with your head misaligned.
Other advantages of standing square vs bladed:
- If your body is bladed with your hips facing to the right, then you're going to have a harder time tracking a target moving to the left. This is because you're starting out with your upper body already twisted to the left, so if you twist more to the left it becomes really uncomfortable. If you start square, you can twist to either side a certain amount if you had to.
- If you're wearing body armor, standing square has obvious advantages.
- Another thing you can do to reduce muzzle rise is to put your support hand farther forward on the forearm of the long gun. This helps reduce muzzle rise and gives a bit finer control when moving the muzzle around(like how when you write with a pen, you hold the pen close to the tip). Holding the forearm farther out does have the disadvantage that it's a bit less stable, since it's farther away from you, the rifle feels heavier due to leverage. So for fighting, it's useful to stand square and hold your support hand near the muzzle for fine control of the muzzle and good control of recoil and muzzle rise for rapid shooting. For target shooting, it's better to stand sideways and hold the forearm very close to your body with your support arm, with your support elbow resting against your hip for stability, since you only care about stability and not about moving the muzzle from target to target, nor about muzzle rise.
- Standing bladed may make your body a smaller target, but any hit on the target will probably go through two or more vital organs. Standing square, a hit is probably going to go through one at most. I wouldn't say either square or bladed has an advantage in this regard.
For more technique tips: http://jumpthestack.blogspot.com/search/label/shooting%20tips