Internal Ballistics | Forensic Ballistics Notes | Budding Forensic Expert

Internal Ballistics

Internal ballistics is the study which deals with the motion of projectile(s) in the bore of the weapon whereas external ballistics deals with the flight from the muzzle of the weapon to the target.

It commences as soon as the first grain of propellant is ignited and proceeds till the projectile leaves the muzzle of the weapon. The study includes all the details concerning the impulse which makes the projectile move out from the muzzle into the air.

As a matter of fact, the projectile obtains its energy during the period it remains in the firearm. This period can be divided as follows:

1. Lock Time
2. Ignition Time
3. Barrel Time

1. Lock Time

The lock time is the period of time between the release of the sear and the striker's collision with the percussion cap. Rapid fire is more favourable with a short lock time. There are several techniques to measure the lock time, and one such system makes use of linear motion sensors and an oscilloscope.

2. Ignition Time

The ignition time is the amount of time that passes between the firing pin striking and the first grain of powder igniting. In typical circumstances, ignition takes place in about 0.002 seconds.

3. Barrel Time

The amount of time between pulling the trigger and the bullet leaving the muzzle end is known as the barrel time. In most weapons, Lock time + Ignition time + Barrel time varies from 0.003 to 0.007 seconds.

Phenomenon of Internal Ballistics

1. Ignition

When the firing pin strikes the primer, the priming compound explodes quickly, creating a jet of flame with very high temperature that enters the propellant chamber through the flash hole. This jet of flame (≈2000°C) ignites the propellant, which burns rapidly to produce a large volume of high-pressure gas and accelerate the bullet down the barrel and out the muzzle end.

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2. Burning of Propellant

Nitrocellulose propellants will burn gently if lit in an open area. If confined, the heat and pressure exponentially increase the burning rate.

Products of combustion of nitroglycerine-based propellants:
Nitroglycerine → Carbon dioxide + Water vapours + Oxides of Nitrogen + Nitrogen + Heat

At normal temperature (0°C) and pressure (760 mm), 1 gm of nitroglycerine produces ~1000 cm³ of gas and generates ~1000 calories of heat. Temperature may exceed 3000°C. Only a fraction of this energy becomes kinetic energy of the projectile; most is wasted.

Progressive Powder: Pressure builds gradually (e.g., multi-perforated grains).
Degressive Powder: Burning rate decreases (non-perforated grains).
Constant Burning Propellant: Single-perforated grains maintain nearly constant surface area.

3. Geometry of Gunpowder

• Combustion: Slow burning in open air with heat dissipation.
• Deflagration: Rapid burning under confinement.
• Detonation: Extremely rapid explosion with shock wave (not typical for propellants).

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4. Combustion of Propellants and Barrel Length

Fast-burning powders are used in short-barrel weapons (pistols/revolvers); slow-burning powders in long-barrel rifles to maintain sustained pressure and achieve higher velocity.

5. Atmospheric Temperature

Extreme temperatures affect pressure and velocity. Indian Ordnance Factories design ammunition for –52°C to +72°C. Velocity changes ≈1 m/s per °C.

6. Shape of the Cartridge Case

Abrupt neck junction improves uniform ignition and complete combustion.

7. Heat Problems & Combustion of Propellants

Gas temperatures reach ~3000°C, but contact time is ~0.001 s, so barrels don’t melt instantly. Continuous firing causes erosion (washing), especially with nitroglycerine powders.

8. Density of Loading

Loading density = (volume of powder / volume of case) × 100. Rifle cartridges: 75–95%. High density → uniform burning and consistent velocity.

9. Barrel Fouling

Metal melting and deposition increases bore diameter, reduces accuracy and velocity.

10. Gas Cutting

Hot gas jets erode grooves in rifling due to projectile deformation or leade erosion.

11. Vibration and Jump

Recoil causes muzzle rise (“jump”) and vibrations, affecting point of impact.

12. Theory of Recoil

Barrel position above the hand causes rotation; even 0.001 s in-barrel time is enough to raise point of impact noticeably at distance.

References

1. Firearms and Ballistics – Brian Heard
2. Internal Ballistics-II – EPG Pathshala
3. Class Notes (NFSU – Delhi)

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