Okay, I KNOW they DO use .50s as anti-Materiel as well as anti-personnel weapons (on occasion)... and believfe it or not, the weapon's crew actually determines to a large extent the the type, number of rounds, and how they are carried. Some carry the magazines loose in large pouches, others carry them in various pockets or have bandoleers made. After all that is said and done, the .50 gets a max of about 70 rounds, which is considered adequate for it's purpose.
The difference between that and what we are talking about is that, according to your statements, you've got this notion that every man in a squad would be using .50s if they did more damage. That means you'd have the entire squad going out with that 70 rounds each instead of the hundreds of rounds of 7.62 or 5.56 or whate4ver that are carried in combat as it is now. There is a huge difference in having one man with a very limited amount of ammo for a specific task than having an entire squad with limited ammo for general purposes.
So, as long as we're on that topic, how about this... You do know that by treaty, the military uses a specific type of ammunition BECAUSE it's less lethal, right? Dum-Dums (soft tip or hollow-core bullets, or generally any bullet that expands on impact) were outlawed for military use by the Hague convention. FMJ is used BECAUSE it is less lethal. Given that, I'm not sure it's logical to assume that because a weapon is more lethal it will be used across the board as a general tool.
Moving on, we have at least one more area that we haven't discussed at all, and that is cavitation. Unlike stories about shock waves from bullets passing nearby, or "hydrostatic shock" cavitation is a known and verifiable effect. The basic scenario is that when a bullet passes through human flesh (and this is where you screwdriver comparison falls apart, by the way) it leaves in its wake a sort of tunnel that is opened as a result of the simple fact that the bullet first pushes material like flesh and blood out of its way, then it takes a second or so for the path to close behind it.
Since it's not been mentioned at all, I have to presume you don't know that the "little tunnel" is not the same diameter as the bullet... and in fact it can be up to 30 times the diameter of the bullet itself because because of the force with which high velocity bullets push flessh and blood out of the way. So, imagine that scenario for a moment. A bullet passes through a body leaving an open "tunnel" behind it , and the more high velocity, the more kinetic energy is used to displace that flesh and blood.
Now consider that the .50 BMG round is more or less half an inch in diameter, and think of what that means for a tunnel 30 times that size... a hole, closing almost instantly, but 15 inches in diameter...
However, it's not the hole itself that is the focus of "bullet forensics" on this matter. It is the cavitation... the closing of the would that sets up shock waves that damage or destroy nearby organs. How nearby do they have to be, you say? Well, one can be fairly certain that a kidney, heart, lung, liver, etc., that is in the primary cavitation area itself is sure to effected. Then there is the matter of blood pressure variances throughout the entire body resulting from that momentary spike of blood trying to find a place to go. Where the bullet hits is all-important to determining damage from cavitation, but the simple fact is that the higher the velocity, the more extreme the cavitation...