Forensic Ballistics – Rapid Review

Cartridge Components

• Case, primer (detonator), propellant (gun-powder), projectile (bullet/shot)
• Shotgun shell extras: wad, card discs, base & percussion cap

Propellants

• Black powder: $75\%$ $KNO_3$, $15\%$ charcoal, $10\%$ sulphur
• Smokeless powder
  • Single-base – nitrocellulose • Double-base – nitrocellulose + nitroglycerine • Triple-base – + nitro-guanidine
• Semi-smokeless: $80\%$ black + $20\%$ smokeless

Wad (smooth-bore only)

• Soft material (felt, cork, plastic, straw, rug)
• Shapes: disc, cup, cushion
• Functions: separates shot ↔ powder, seals bore, acts as piston, prevents gas escape, lubricates (grease)

Rifled Firearms & Rifling

• Spiral grooves cut by broaching; raised = land, hollow = groove
• Typical grooves: $4\text{–}7$
• Purpose: gyroscopic spin → straight flight, penetration, stability

Choking (shotgun)

• Terminal $\sim$ few cm muzzle constricted (full, modified, improved, cylinder)
• Reduces shot spread, ↑ velocity

Calibre / Gauge

• Rifled: distance between opposite lands (in mm or inches)
• Smooth-bore gauge: number of lead balls per $1$ lb whose diameter fits the bore (e.g., $12$-gauge)

Key Range Indicators (rifled weapons)

• Flame: to $7.5\text{ cm}$ (pistol) / $15\text{ cm}$ (rifle)
• Smoke: to $15\text{ cm}$
• Unburnt powder: up to $60\text{ cm}$ (handgun) / $60\text{–}100\text{ cm}$ (rifle)

Entry-Wound Hallmarks

• Abrasion/contusion collar (outer) – indicates direction
• Grease/dirt collar (inner)
• Possible singeing, blackening (smudging), tattooing (stippling), lead ring, cherry-red tissue (CO)

Range-Specific Wounds (rifled)

• Contact: cruciate skull holes, muzzle imprint, cherry red tissues; burning/blackening minimal externally; back-spatter
• Close (within flame): circular hole + scorching, singeing, smudging, collars, tattooing
• Near (powder only, $10\text{–}50\text{ cm}$): collars, smudging + tattooing, no singeing
• Distant (> powder range, >$1$ m): only grease + abrasion collars

Exit Wounds (rifled)

• Larger, everted edges, no collars, no burning/blackening/tattooing; variable shape

Shotgun Injury Ranges (smooth-bore)

• Contact/near-contact (≤$30\text{ cm}$): single round defect ≈ bore, minimal soot; tight contact → muzzle imprint; possible stellate if bone underneath
• Close (<$1\text{ m}$): singeing/blackening/tattooing; $30\text{–}60\text{ cm}$ – wad abrasions (four-petal); rim scalloping (“rat-hole”)
• Short range $1\text{–}2\text{ m}$: single $4\text{–}5\text{ cm}$ aperture, no soot/tattoo
• Intermediate $2\text{–}4\text{ m}$: central hole shrinks, multiple satellite pellet holes
• Distant >$4\text{ m}$: separate pellet wounds over $10\text{–}15\text{ cm}$ spread; wad may travel $2\text{–}5\text{ m}$
• Pellet dispersion: diameter (in inches) ≈ range (in yards) × $1.5$

Shotgun Exit Wounds

• Usually absent; occur with contact, tangential, thin body parts, buck-shot, slugs

Comparative Entry vs Exit (general)

• Entry: inverted edges, abrasion & grease collars, possible soot/heat effects
• Exit: everted, larger/irregular, clean margins (no collars), no soot/heat, no tattooing

Diagnostic Checklist

• Presence & type of collars → confirms entry & direction
• Soot (smudging) → within smoke range
• Tattooing → within powder range (no flame needed)
• Singeing/burning → within flame range
• Wad marks → shotgun within <5 m
• Back-spatter inside barrel → contact shot

Scene & Manner Clues

• Multiple wounds with single trigger pull → semi-automatic capability
• Undisturbed locked room + upward/accessible wounds → suggest suicide