Internal ballistics happens inside the barrel: expanding propellant gas accelerates the bullet while rifling grooves spin it up. That spin gyroscopically stabilises the bullet in flight — without it the bullet would tumble.
Spin rate: ω = (v / t) · 2π t = rifling twist rate (turns per metre)
External ballistics (drag + gravity):
m·dv/dt = −½·ρ·Cd·A·|v|·v + m·g + F_wind
Drop at range R (no-drag approx.): Δy ≈ −g·(R / (v·cosθ))² / 2
Gunshot residue (GSR) distance bands, muzzle → target:
Contact < 2 cm muzzle stamp, searing, heavy soot
Close range 2–60 cm dense soot + stippling (burnt/unburnt powder)
Intermediate 0.6–2 m scattered stippling, no soot
Distant > 2 m no stippling — wound size set by caliber alone
- Caliber — sets muzzle velocity, bullet mass and rifling twist, which change drop, time of flight and spin rate.
- Elevation angle — the barrel's aim above horizontal; higher angles trade drop for range.
- Crosswind — deflects the bullet sideways over its flight time; heavier, faster bullets resist it better.
- Muzzle-to-target distance — drives the live GSR classification, the same distance bands examiners use to estimate firing range from a wound or garment.
Real forensic examiners match these three sub-fields to physical evidence: rifling marks on a recovered bullet can identify the specific gun barrel (internal ballistics), the trajectory and drop reconstruct the shooter's position (external ballistics), and GSR/wound patterns estimate firing distance (terminal ballistics).