Exam 13: Firearms

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Chemical tests to detect and identify propellants and explosives used in firearms are crucial in forensic science, particularly in the investigation of crimes involving firearms. These tests can determine the presence of specific substances that are commonly used in the manufacture of ammunition and explosive devices. Here is a list of some chemical tests and analytical techniques used for this purpose:

1. **Modified Griess Test**: This test is used to detect the presence of nitrite ions, which are a common byproduct of the combustion of gunpowder. A positive result indicates the presence of nitrate or nitrite residues, which are typical in gunshot residues (GSR).

2. **Sodium Rhodizonate Test**: This test is sensitive to lead, which is a component of many primers used in ammunition. When the sodium rhodizonate reagent is applied to a sample containing lead residues, a purple or pink coloration will develop.

3. **Diphenylamine Test**: This test is used to detect the presence of nitrates, which are a key component of most gunpowders. A blue coloration after the application of the diphenylamine reagent indicates a positive result for nitrates.

4. **Thin Layer Chromatography (TLC)**: TLC can separate the components of gunpowder and other explosives based on their different rates of movement on a special paper or gel under the influence of a solvent. This can help identify specific compounds present in the sample.

5. **Gas Chromatography-Mass Spectrometry (GC-MS)**: GC-MS is a sophisticated analytical technique that can separate and identify the individual components of complex mixtures. It is highly sensitive and can detect and identify a wide range of organic and some inorganic explosives and propellants.

6. **High-Performance Liquid Chromatography (HPLC)**: HPLC is another advanced technique used to separate, identify, and quantify the components of propellants and explosives. It is particularly useful for analyzing nitroglycerin, nitrocellulose, and other stabilizers in propellants.

7. **Fourier-Transform Infrared Spectroscopy (FTIR)**: FTIR can identify organic and some inorganic compounds based on their characteristic absorption of infrared light. It is useful for identifying binders and plasticizers in explosives.

8. **Raman Spectroscopy**: This technique uses the scattering of laser light to identify molecular vibrations, providing a fingerprint for different chemical substances. It is non-destructive and can be used to identify inorganic oxidizers and organic fuels in explosives.

9. **X-Ray Diffraction (XRD)**: XRD can be used to identify crystalline materials by looking at the pattern of X-rays scattered by the sample. It is useful for identifying inorganic oxidizers in explosives.

10. **Ion Mobility Spectrometry (IMS)**: IMS is a rapid and sensitive technique often used in portable devices for on-site detection of explosives. It separates ions based on their mobility in an electric field.

11. **Microscopic Analysis**: While not a chemical test per se, microscopic examination can reveal the physical characteristics of gunpowder particles and other residues, which can be indicative of the type of ammunition used.

These tests and techniques are often used in combination to confirm the presence and identify the type of propellants and explosives involved in firearm-related incidents. The choice of test may depend on the nature of the sample, the suspected components, and the resources available to the investigators.

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Shotgun ammunition and revolver ammunition differ significantly in design, purpose, and the resulting markings they leave on spent casings or shells.

**Shotgun Ammunition:**
Shotgun ammunition, commonly known as shotgun shells, is designed to fire a number of small projectiles, called shot, or a single projectile, called a slug. Shotgun shells consist of a plastic casing with a brass base containing the primer. Inside the shell, there is typically a plastic wad that separates the shot or slug from the gunpowder. When fired, the expanding gases from the ignited gunpowder propel the wad and shot or slug down the barrel.

The markings on spent shotgun shells can include indentations from the firing pin, which strikes the primer to ignite the gunpowder. The brass base may also show extractor or ejector marks from the shotgun's mechanism as it removes the spent shell. However, because the barrel of a shotgun is usually smooth (unless it's a rifled shotgun barrel designed for slugs), there are no rifling marks on the shot or wad as they exit the barrel.

**Revolver Ammunition:**
Revolver ammunition, on the other hand, is typically a metallic cartridge that contains a single projectile, known as a bullet, which is seated in a cylindrical brass casing containing the primer and gunpowder. When the revolver's hammer strikes the primer, the gunpowder ignites, and the bullet is propelled through the barrel.

The markings on spent revolver ammunition are more distinctive than those on shotgun shells. The firing pin leaves a mark on the primer, and the cartridge casing may have impressions from the revolver's cylinder, extractor, and ejector. Most importantly, the bullet will bear the unique rifling marks from the barrel. Rifling refers to the spiral grooves cut into the barrel's interior, which impart a spin to the bullet for stability and accuracy. These grooves leave corresponding marks on the bullet's surface, which can be matched to the firearm that fired it.

In summary, shotgun shells are designed to fire multiple projectiles or a single slug and typically do not leave rifling marks, while revolver ammunition fires a single bullet that will bear the unique rifling marks of the barrel it was fired from. The spent casings of both types of ammunition will show firing pin impressions and possibly extractor or ejector marks, but the revolver casings may also show additional marks from the cylinder where they were held during firing.