There's a reason that the M2 .50-caliber machine gun design has endured since John Browning first created it 100 years ago, in 1918: The mechanical reliability of the weapon and ballistics of the round are still exactly what a soldier needs to kill large numbers of people and light vehicles quickly at long range.
Here's how it works and how it affects a human body.
First, the M2 and its ammunition can be legally used to target enemy personnel, despite apersistent myth that states it can only be aimed at equipment. That said, it isn't designed solely for anti-personnel use. An anti-personnel specific weapon usually has smaller rounds that are more likely to tumble when they strike human flesh.
See, there are three major effects from a metal round hitting flesh that are likely to cause severe injury or death. First, there's the laceration and crushing from the round's traversalthrough the flesh.
Then, there's the cavitation,which has two parts. The first cavity is the permanent one:the open space left from the laceration discussed above. But there's a second, temporary cavity. As the round travels through the body, it's crushing the flesh and pushing it out of the way very quickly. That flesh maintains its momentum for a fraction of a second, billowing out from the path of the bullet. The flesh can tear and cells can burst as the tissue erupts outward and then slams back.
In the above GIF of ballistics gel taking a .50-cal. round, you can see all three effects. There's the laceration and crushing immediately around the bullet, the huge cavity as the gel flies apart, and the shockwave from that expansion as it forces the gel to fly outwards before re-compressing. The cavitation and re-compression is so violent that you can see a small explosion in the first block from the compressing air.
Finally, there's the shock wave. That temporary cavity discussed above? The flesh all around it is obviously compressed as the cavity expands, and that's where the shock wave starts. The cavity pushes outward, compressing the flesh and the energy in the compressed flesh keeps traveling outward until it dissipates. This can also cause separations and tears. In extreme situations, it can even cause damage to nerve tissue, like the spinal cord and brain.
Typical rifle rounds generally aim to maximize the first two effects, laceration and crushing and cavitation. A relatively short, small round — 5.56mm or .223 caliber in the case of the M16 — travels very quickly to the target. When it hits, it quickly begins to yaw and then tumble, depositing all of its kinetic energy to create a large, temporary cavity. And the tumble of the round allows it to crush and cut a little more flesh than it would if flying straight.
But maximizing design for cavitation is maximizing for tumble, and that can make the round more susceptible to environmental effects in flight, making it less accurate at long range.
But Browning wanted the M2 to be accurate at long ranges, so he opted for a big, heavy round with a sharp tip. That's great for flying long ranges and punching through the skin of a vehicle, but it can cause the bullet to punch right through human flesh without depositing much kinetic energy, meaning that it only damages the flesh directly in the path of the round.
But there's a way to still get the round to cause lots of damage, even if it's going to pass right through the enemy: maximizing its speed and size so that it still sends a lot of energy into the surrounding flesh, making a large cavity and creating a stunning shockwave. Basically, it doesn't matter that the round only deposits a fraction of its energy if it has a ton of energy.
The M2 fires rounds at a lower muzzle velocity than the M16 and at similar speeds to the M4, but its round is much larger and heavier. The M33 ball ammo for the M2 weighs almost 46 grams, while the M16's NATO standard 5.56mm round weighs less than 4 grams. That means, flying at the same speeds, the M2 .50-cal. has 11 times as much energy to impart.
It also maintains more speed during flight. So, when the M33 round from the M2 hits a target, it does usually pass through with plenty of its kinetic energy left with the exiting round. But it still cuts a massive path through its target, doing plenty of damage from the first effect. And it compresses plenty of flesh around it as it forces its way through the target, creating a large permanent cavity and a still-impressive, temporary cavity.
But it really shines when it comes to shock wave damage. The M33 and other .50-cal. rounds have so much energy that even depositing a small fraction of it into the surrounding tissues can cause it to greatly compress and then expand. With a large round traveling at such high speeds, the shock wave can become large enough to cause neurological damage.
Yeah, the target's flesh deforms so quickly that the energy can compress nerves or displace them, shredding the connections between them and potentially causing a concussion.
And all of that is without the round hitting a bone, which instantly makes the whole problem much worse for the target. All rounds impart some of their energy to a bone if they strike it, but with smaller rounds, there's not all that much energy. With a .50-cal, it can make the bone explode into multiple shards that are all flying with the speed of a low-velocity bullet.
The M2 can turn its target's skeleton into a shotgun blast taking place inside their body. The harder the bone that takes the hit, the more energy is imparted to the skeleton before the bone breaks. On really hard bones, like the hip socket, the huge, fast-moving round can leave all or most of its energy in the bone and connected flesh.
This will basically liquefy the enemy it hits as the energy travels through the nearby muscles and the organs in the abdominal cavity. There's really no way to survive a .50-cal. round if it hits a good, hard, well-connected bone. Not that your chances are much better if it hits anything but an extremity.
In fact, the .50-cal. hits with so much energy that it would likely kill you even if your body armor could stop it. The impact of the armor plate hitting your rib cage would be like taking a hit from Thor's Hammer. That energy would still crush your organs and break apart your blood vessels and arteries, it would just allow your skin to keep most of the goop inside as you died. No laceration or cavitation, but so much crushing and shock wave that it wouldn't matter.
So, try to avoid enemy .50-cal. rounds if you can, but rest confident in the effects on the enemy if you're firing it at them. The ammo cans might be super heavy, but causing these kinds of effects at over a mile is often worth it.
There are a lot of vets sharing their stories of bodies hit by .50-cal. rounds on Quora, if you're into that sort of thing.