Internal, External and Terminal Ballistics: Basics

Ballistics is split by event, not by sub-discipline. The boundaries are physical: the bullet crossing the muzzle separates internal from external, and the bullet contacting the target separates external from terminal. Internal runs about 1 to 2 ms, external runs from a few ms to several seconds, terminal is again of the order of milliseconds.

Correctly assigning each phenomenon to its stage is the foundation of the topic: recoil belongs to internal ballistics, Magnus effect to external, and permanent cavity to terminal.

Internal ballistics begins when the firing pin strikes the primer cup and ends when the base of the bullet crosses the muzzle. The whole pipeline takes about 1 to 2 milliseconds.

Primer ignition. The firing pin crushes the primer cup against the anvil, detonating the shock-sensitive priming compound. The resulting flash crosses the flash hole and lights the main propellant charge. The detailed primer chemistry is covered in the gunshot residue analysisbullet; here, the only fact examiners test is that priming is shock-sensitive and the main charge is heat-sensitive.

Propellant deflagration. Smokeless powder burns by deflagration, a subsonic combustion wave, not by detonation. Deflagration produces a controlled rising pressure curve that pushes the bullet, while detonation would burst the case and chamber. Black powder also deflagrates, with more solid residue.

Pressure-time curve. Chamber pressure rises rapidly to a peak, then falls as the bullet moves down the barrel and the volume behind it grows. Peak pressure is roughly 35,000 psi for a handgun cartridge such as 9 mm Para, and roughly 50,000 to 62,000 psi for a rifle cartridge such as 7.62 mm NATO. BIS and SAAMI specifications quote a maximum average pressure that the cartridge must not exceed in proof firing.

Bullet engraving and barrel time. As the bullet starts moving, the rifling lands cut into its jacket, producing the lands-and-grooves engraving that the comparison microscopelater uses for individualisation. Engraving adds friction, which is why peak pressure happens early. Barrel time, the interval from primer strike to muzzle exit, is around 1 to 2 ms.

Recoil. Conservation of momentum: the impulse on the bullet equals the impulse on the shooter, in opposite directions. The standard form isF·Δt = m·v. A 9 mm 8 g bullet leaving the muzzle at 350 m/s carries momentum 2.8 kg·m/s, and the shooter (mass plus pistol) absorbs the same momentum in the opposite direction. Barrel harmonics, the standing-wave vibration set up during the burn, controls where the muzzle points at the instant of bullet exit and explains why two identical rifles can group differently.

External ballistics begins at muzzle exit and ends at target contact. In vacuum, the trajectory is a clean parabola. In real air, four forces and one rotation-effect distort that parabola.

Gravity. Pulls the bullet downward at 9.81 m/s² regardless of velocity. A 9 mm Para round leaving the muzzle horizontally at 350 m/s reaches 100 m in about 0.29 s and drops roughly 33 cm. This is why sights are zeroed at a chosen range and must be held over at longer ranges.

Air drag. The dominant retarding force. At subsonic velocities, drag is proportional to v²; at trans- and supersonic velocities, drag rises sharply through the transonic region as the bullet pushes against its own shock wave. Drag is summarised in the ballistic coefficient (BC), a dimensionless ratio of the bullet's sectional density to the drag coefficient of a reference projectile (G1 or G7 form). A higher BC means flatter trajectory, less wind drift and more retained energy.

Wind drift. A crosswind pushes the bullet laterally throughout its flight, with drift growing roughly with the square of the time of flight.

Spin-stabilisation. Rifling spins the bullet about its long axis at tens of thousands of rpm. The resulting gyroscopic moment keeps the nose pointing forward against the pitching couple that air drag would otherwise apply. Smoothbore shotguns rely on shot mass and spread, not spin, which is why slug accuracy from a smoothbore is poor beyond about 75 m.

Magnus effect. A spinning bullet in a crosswind generates a small lift or sink force perpendicular to both the spin axis and the wind direction. The effect is modest at pistol ranges and significant for high-BC match bullets at long rifle ranges. At long rifle ranges, shooters correct for the Magnus effect in dope sheets.

Coriolis force. The Earth's rotation deflects a flying bullet, to the right in the northern hemisphere. Negligible at pistol ranges, measurable past about 1,000 m, and significant for sniper engagements past 1,500 m. Army Ordnance Corps firing tables and DRDO ARDE Pune small-arms range data include Coriolis corrections for long-range trials.

The combined trajectory is therefore not a clean parabola. It is asymmetric: the descent is steeper than the ascent because drag has been bleeding velocity throughout the flight. The maximum range angle in air is about 30 to 35 degrees, not 45 degrees as it would be in vacuum.

Terminal ballistics begins at target contact. For wound ballistics, the international standard test medium is 10 percent ordnance gelatin at 4 °C, which approximates the density and elastic response of muscle tissue. The FBI 10-shot protocol and the Fackler wound-profile diagrams are the standard reference framework for wound ballistic evaluation.

Penetration depth. The straight-line distance the bullet travels through the medium before stopping. FBI service-ammunition criteria require 12 to 18 inch (30 to 46 cm) penetration in calibrated gelatin for handgun duty. DRDO ARDE Pune and the Army Small Arms Wing run similar gelatin trials for Indian service rounds.

Permanent cavity. Tissue destroyed by direct bullet contact along the wound path. For an FMJ that does not deform, the permanent cavity is essentially a bullet-diameter channel. For an expanded JHP, the mushroom diameter (often 1.5 to 1.8 times the original calibre) doubles or trebles the cross-section.

Temporary cavity. Radial stretching of tissue away from the wound path, driven by the pressure wave as the bullet transfers kinetic energy. The cavity expands within milliseconds and collapses within tens of milliseconds. In high-velocity rifle rounds, temporary cavity diameter can reach 5 to 10 times bullet calibre. It is responsible for what older texts call "hydrostatic shock", especially in fluid-filled organs.

Bullet behaviour on impact. Three modes are tested.

• Expansion. JHP and soft-point hunting bullets mushroom on impact, increasing frontal area and slowing rapidly. Result: large permanent cavity, shallower penetration.
• Fragmentation. Military 5.56 mm NATO bullets tend to yaw, fragment at the cannelure and disperse, creating multiple wound tracks.
• Full penetration. FMJ rounds (the Hague-Convention-compliant military pistol standard) pass through without deforming, transferring less energy but penetrating more.

Yaw and tumble. Long, thin rifle bullets (5.56 mm and 7.62 mm) are spin-stabilised in air but only conditionally stable in dense tissue. Once the nose dips a few degrees in the target, the bullet tumbles end-over-end, dumping a large fraction of its kinetic energy into the temporary cavity. This is the mechanism behind the disproportionately large wounds seen with high-velocity small-calibre military rounds.

Energy transfer. Kinetic energy delivered to the target equals½m(v_impact² − v_exit²). For a bullet that lodges in the target, v_exit is zero and the entire impact KE is dumped. Tissues vary in vulnerability: brain and liver (fluid-rich) suffer disproportionately from cavitation; lung (air-filled) absorbs less.

Stopping power versus penetration. The trade-off examiners like to test. JHP gives high energy transfer in shallow penetration; FMJ gives deep penetration with low energy transfer. Indian state police duty rounds and BSF / ITBP service rounds are specified on this trade-off.

The entry-track-exit description is the core wound ballistics summary. The entry wound is typically round, with an abrasion collar (a 1 to 3 mm ring of friction-scraped epidermis around the perforation) and, at close range, soot blackening and powder tattooing covered in the range determinationbullet. The wound track shows the permanent cavity. The exit wound is typically larger and more irregular, with everted edges, because the bullet has deformed and may be tumbling.

Indian institutional anchors: DRDO ARDE Pune runs small-arms ballistic trials for service ammunition; the Army Ordnance Corps proof firing at Khamaria verifies pressure and velocity to BIS specifications for lot acceptance; CFSL Chandigarh and CFSL Hyderabad ballistics divisions handle casework beyond state SFSL capacity; BSF and ITBP procurement teams specify ballistic protective gear by NIJ standard levels (II and IIIA for soft armour, III and IV for hard plates).

The primary reference frameworks are the FBI ballistic gelatin protocol (source of the 12 to 18 inch penetration window for duty ammunition) and the wound profiles published by Martin Fackler at the Letterman Army Institute, which validated ordnance gelatin against pig-flank tissue as a muscle simulant.