Apart from various gases four types of particles have been detected in firearm discharge residue:
1. Spherical particles
2. Irregular particles
3. Clusters of particles
As a general rule greater than 70% of the particles are spherical. They may be perfect spheres or they may be distorted in some way. Three-dimensional roundness is the basis for this classification. The surface of the particles may be smooth, fuzzy, scaly, or have smaller spheres on their surface. Sometimes they are perforated, capped, broken, or stemmed. The vast majority have diameters of less than 5 ^m although they range in size from <0.5 ^m to >32 ^m (it is not practical to detect particles less than 0.5 ^m in diameter).
The irregular particles constitute up to about 30% of the total particle population. The size varies over a wide range from <1 ^m to several hundred microns. The larger of the irregular particles sometimes have some small spherical particles attached to them. The irregular particles have the same compositions as the spherical particles.
None of the particles exhibits any features that would suggest a crystal or mineral origin, such as straight or sharp edges, and they are frequently squashed or flattened in appearance.
Clusters consist of from five to several hundred spherical particles attached to each other in a similar fashion to a bunch of grapes. They occur infrequently and appear to be a product of high power or high velocity. Flakes of smokeless powder are few in number and are occasionally seen in promptly collected residue. Unlike the other three types of particles, which are inorganic in nature, the flakes are organic although sometimes spherical particles are embedded in their surface. They range in size from about 50 to 1,000 ^m. Clusters and powder flakes are rarely seen in casework as they are usually relatively large in size and would be lost rapidly from skin and clothing surfaces.
Spherical particles are thought to result by rapid condensation from a vapor whereas the irregular particles may be produced by solidification of droplets of molten material that are flung against the inside surfaces of the firearm.
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