First, bullet caliber has very little to do with range, otherwise civil war era bullets would have much longer range than either of the two rifles. So what does really matter for bullet range? There are a number of factors, such as:
- Weight of the bullet (which affects momentum and how fast a bullet travels)
- Amount of propellant in the cartridge.
- Quality of propellant in the cartridge. Smokeless powders provide more thrust than the equivalent amount of black powder, for instance.
- Shape of the bullet (ball, spitzer bullet etc.)
- Amount of rifling or spin imparted on the bullet (which affects the stability of the bullet in the air) and its aerodynamics.
With that said, let us now compare two cartridges, both of which were designed for use with the M16. The first is the M193 cartridge which came out with the original M16 A1 model and the second is the M855 cartridge that came with the improved M16 A2 model. Both cartridges are of the same length (44.70 mm.) The M16 A2 model has a barrel with more rifling, which means the bullet coming out of it spins a lot more than a bullet coming out of an M16 A1 barrel. The M16A1 barrel has a 1 in 12 twist, whereas a M16A2 has a 1 in 7 rifling twist. Even though the bullets of both cartridges are of the same diameter (5.70 mm.), the length of the bullets differ. The bullet from the M193 cartridge is shorter and it weighs about 3.56 gm. whereas the bullet from the M855 is longer and weighs about 4.0 gm. The overall length of both cartridges is the same though at 44.70 mm.
Now let us compare the performance of these two cartridges from the table below:
5.56 NATO Ball Ammunition Ballistic Comparison based on Aberdeen Proving Ground Data velocity (fps) trajectory (in.) drop (inches) drift (inches)* range M193 M855 M193 M855 M193 M855 M193 M855 (meters) 0 3,200 3,100 -2.5 -2.5 0.0 0.0 0.0 0.0 100 2,774 2,751 +2.8 +4.4 -2.2 -2.3 1.3 1.1 200 2,374 2,420 +2.7 +5.8 -9.9 -10.2 5.8 4.9 300 2,012 2,115 -4.9 0.0 -25.1 -25.3 14.2 11.8 400 1,680 1,833 -23.0 -15.0 -50.8 -49.5 27.6 22.4 500 1,373 1,569 -56.2 -42.9 -91.6 -86.7 47.5 38.0 600 1,106 1,323 -113.1 -88.2 -156.1 -141.3 76.4 59.5 700 995 1,106 -206.8 -156.1 -257.3 -220.9 113.5 88.4 800 927 1,010 -339.9 -267.7 -398.0 -339.2 156.1 124.9 * Drift for 10 mph wind. M193 Ball ammunition fired in M16A1 rifle with 250 meter battle sight zero. M855 Ball ammunition fired in M16A2 rifle with 300 meter battle sight zero.First thing we notice from the table above is that the bullet from an M193 cartridge initially comes out flying at a faster velocity than the M855 cartridge (3200 fps vs. 3100 fps), which is to be expected, since it is a lighter bullet, and also because the bullet from the M855 is spinning about its axis a lot faster since the rifling on an M16 A2 is more than on an M16 A1. Therefore, a naive person might conclude that the M193 bullet has greater range since it is travelling faster.
However, this is not the end of the story. Look at the velocities once the bullets have passed 200 meters. Suddenly the M855 bullet is moving faster. What is the cause of this? It is because of the spin on the bullet. The M193 bullet is spinning about its axis less and therefore starts to wobble and lose stability in the air earlier. Hence, it encounters more air turbulence and starts to lose its velocity faster. In fact, by the time the bullets have traveled 200 meters, it has lost so much of its velocity that it is now moving slower than the M855 bullet. The situation only gets worse as it travels 300 meters, 400 meters etc. Suddenly, it looks like the M855 bullet may have greater range.
Also observe the drop columns for the two cartridges. Note that up to a distance of 300 meters, there is little difference in the height that these two bullets drop. However, at 400 meters, the bullet from the M193 has dropped 50.8 inches to the M855's 49.5 inches, which is a difference of over an inch. By 500 meters, the difference is almost 5 inches, by 600 meters, the difference is almost 15 inches and at 700 meters, the difference is over 36 inches! So why does one bullet drop to the ground more than the other? Again, this has to do with the bullet velocity. The M193 bullet takes a longer time to reach 500 meters distance than the M855 bullet, because it is moving slower. That means that the force of gravity has a longer time to act on it. Think about it this way: say that it takes 1 second for the M855 bullet to reach 500 meters and 1.1 seconds for the M193 bullet to reach the same distance, since it is moving slower. Now the same force of gravity is acting on both bullets and pulling them towards the earth at the same rate. However, since gravity has only acted for 1 second on the M855 bullet, whereas it has acted for 1.1 seconds on the M193 bullet, therefore the M193 bullet will be pulled closer to the ground. What this means is that if both bullets are fired from the same initial height, beyond 300 meters or so, the M193 bullet will strike the ground first and therefore have lesser range.
Also note the drift columns on the right of the table. Due to the greater spin of the M855 bullet, it is more stable in the air and tends to drift a lot less than the M193 bullet and therefore has more accuracy.
Now let us get back to the initial question about which has greater range, the AK-47 or the M-16. Now these are rifles of different calibers. Let us say that for the sake of argument, we will assume an M16 A2 which uses the M855 cartridge we examined earlier. We will also assume that the AK-47 uses the M43 cartridge, which is one of the more common cartridges used for this weapon. As we know, the AK-47 has a 7.62 mm. diameter barrel, compared to the M-16's 5.56 mm. diameter barrel, therefore the AK-47's bullet has a larger diameter and is also a heavier bullet. The M43 cartridge is 39 mm. long and has a bullet diameter of 7.92 mm. The bullet also weighs between 8 gm. and 10 gm. depending on type of bullet, which means it is much heavier than the M855 bullet, which tops out at 4 gm. The two bullets also have different amounts and types of propellants, which means the velocities of the respective bullets will be affected by this factor as well.
When the M43 bullet is fired, it carries greater momentum than an M855 bullet. This is because of its greater mass. This means that the M43 bullet packs a bigger punch than an M855 bullet because of more momentum. Remember though that momentum = mass * velocity. Even though the M43 bullet has greater momentum, it actually comes out of the barrel slower than an M855 bullet (710 meters/sec. vs. 940 meters/sec.) This means that it initially appears that the bullet from an M855 will travel further than the M43 bullet. But as we've seen previously, initial velocities don't necessarily tell the whole tale. So let us examine what happens after the bullets from both rifles travel a bit of distance.
Unfortunately, after the bullets have travelled about 300 meters, things begin to actually get worse for the M43 bullet. Due to its aerodynamics, it travels slower than an M855 bullet. This means it will hit the ground sooner than an M855 bullet fired from the same height off the ground, which means lesser range.
Therefore, even though the AK-47 bullet has greater momentum and packs a bigger punch than an M16 A2 bullet, it travels a lot slower and hits the ground at a lesser distance than the M16 A2 bullet. Hence the M16 has a greater range than an AK-47.