For Walkthrough Wednesday, we are continuing our discussion from the previous week on ballistics; specifically, we will be discussing internal ballistics. Our reference for this is Appendix B, TC 3-22.9, Change 1 dated January 2017.
To recap, there are three major types of ballistics: internal, external, and terminal. Today we will be discussing Internal Ballistics.
Internal Ballistics start the moment the primer is struck by the firing pin. The expanded cartridge case, held firmly in place by the chamber walls and the face of the bolt provide rear obturation, keeping the burning propellant and created expanding gasses in front of the cartridge case. As the powder burns, chamber pressure builds, and the projectile is pushed forward through the throat and engages the rifling.
As the Projectile moves down the barrel further, the Rifling in it imparts a rotational spin to the projectile. This provides stability for the round upon exiting the barrel similar to a football thrown from a quarterback.
The stability of the round is determined by the twist rate and is designed to give the optimal amount of stability to a given projectile. Longer, heavier rounds require more spin than lighter, shorter rounds for optimal performance.
If the twist rate is wrong, it will adversely affect the round. If the round is over-stabilized, the projectile will perform poorly for terminal ballistics, as it will punch straight through with minimal disturbance of tissue (e.g. the M855 green tip ammunition). If the projectile is not stabilized enough, it will not travel accurately and will be heavily affected by atmospheric conditions and drag.
These pictures are computer re-creations of what the barrel of a standard rifle goes through as the bullet is passing through it. Notice that the barrel is flexing in more than one direction, as all of these pictures are taking place SIMULTANEOUSLY. This is what is referred to as barrel harmonics. A method of ensuring that the barrel has the same harmonics every time is to isolate it from the handguard so that the firer cannot flex or effect the barrel as it is fired. This is what is referred to when someone says their barrel is ‘free-floated’.
The last part that is affected by Internal Ballistics is when the projectile leaves the crown of the muzzle. If the crown is damaged in any way, it can have an effect on the projectile’s trajectory when it transitions to external ballistics. The analogy used to describe it would be to imagine a quarterback’s hand isn’t in the same place every single time he throws the football, it will go different ways due to inconsistency in the release of the projectile. It’s the same for our projectile. That is why most match-grade or sniper rifles have recessed crowns. It is also important for Paratroopers to understand that scraping the Crown with dental picks (as they are known to do for cleaning) will cause damage and inconsistency to their rifle’s performance.
So to sum up, Internal Ballistics is the study of projectiles from the moment the firing pin strikes the primer, to just before it exits the barrel. The rifling twist rate determines how stable (or unstable) a projectile is. Barrel Harmonics can determine where the projectile will go after transitioning to external ballistics. And one of the most critical things that affects the transition from internal to external ballistics is the crown of the muzzle. Next week, we will discuss various terms that are associated with ballistics.