Time to start a new discussion about Machine Guns. Specifically, we will start to address the specific ballistic considerations that come into play when employing machine guns. We will be having a discussion about the Maximum Ordinate of the round, what the beaten zone is, what the cone of fire is, and danger space. All this information can be found in FM 3-22.68, chapter 5.
Employment considerations for machine guns are a little different than they are for a rifle or pistol. Not only do you have to compensate for the weapons’ movement in the shot process due to the cycling of the bolt and recoil, you have to also account for what those rounds are doing in flight and when they hit their target.
To aid us in our discussion, we will use some terms from the science of ballistics. There are three types of ballistics: Internal, External, and Terminal. Talking about these three could be a lengthy discussion, as there are several doctoral theses written about all of them. I’m going to bring up a few terms that specifically relate to external ballistics so that we can talk about what the projectiles are doing in flight.
The first thing I will discuss will be trajectory. The projectile’s trajectory is what happens when the Paratrooper offsets their line of bore (imagine a laser beam coming straight out the end of your barrel) based off their point of aim to compensate for the effects of gravity, drag and wind. It is simply the path of the round in flight.
The highest point in that path between the muzzle of the machine gun and the threat is known as the Maximum Ordinate, or max ord for short. It generally occurs about 2/3rds the distance between the weapon and the target. Picture two illustrates what this looks like at various distances. From this diagram, we see that the maximum ordinate for a round at 1100 meters is 6 meters (approximately 18 feet off the ground). Why this is important to know will be in a separate discussion.
One thing that machine guns have that rifles or pistols don’t is a cone of fire. A cone of fire, is what happens when a burst is fired out of a machine gun. It is caused by everything from the vibration of the machine gun, to differences in bullet weight, and variations in the atmospheric effects on each projectile.
Where the cone of fire intersects with the ground is called a beaten zone. It is generally represented as an elliptical pattern. Picture three illustrates what it looks like, and what affects it. If the threat is at short range, the beaten zone is long and narrow, as the projectile’s trajectory is shallow and external ballistics have had minimal impact. As the range to the threat increases, the beaten zone shortens and widens. This is due to the projectile assuming more of an arcing trajectory to compensate for the distance, and the external ballistics have had more of an effect on the projectiles making up the burst.
Finally, we are discussing Danger Space. It is the space between the muzzle of the Machine Gun and the threat where the trajectory rises les than 1.8 meters (the height of the average soldier) from the ground. Danger space is generally going to be the entire distance between the Machine gun and threat at ranges of 700 meters and less on uniformly sloping terrain. As we see in picture two, the trajectory of the round past 700 meters on uniform sloping terrain means that the potential exists for only part of that total distance to be danger space. The other consideration for this is dead space in the engagement area. Dead space is any dip in the terrain that the machine gunner cannot see directly into, therefore a threat could possibly exist in. Dead space needs to be covered by indirect fire capabilities such as M320’s or mortars.
So to sum up, we’ve discussed a little about ballistics, what the trajectory is, what max ord is, what the beaten zone is, what the cone of fire is, and what Danger Space is. We will continue our discussion on Machine gun Theory by discussing the classes of fire.