There are a number of different methods by which the rifling may be cut into the barrel of a weapon. A competent forensic firearms examiner should not only be aware of the various methods, but should also be able to identify which method has been used in a particular weapon.
The ability to correctly identify the type of rifling is, in fact, of little or no use to the examiner when carrying out a micro-stria comparison. It is, however, one of those frequently, albeit nonsensical, questions encountered during the qualification of an expert in court.
Rifling process. The actual rifling of a weapon is carried out in a number of stages. Firstly, the weapon is rough bored using a simple drill. It is then reamed to smooth out the roughest of the spiral scratches produced during the drilling.
The barrel is then rifled using one of the methods as listed under Rifling Methods. After rifling, the barrel is then given a final smoothing. The most frequently used methods for this are lead lapping and ball burnishing.
In lead lapping, a lead plug of the same diameter as the bore is repeatedly pulled through the rifling whilst being washed through with a fine abrasive. As the barrel becomes progressively smoother, the fineness of the abrasive is increased. This is the most commonly used method and gives a finish satisfactory for most uses.
Ball burnishing is generally only carried out on high-quality rifles and consists of repeatedly pushing a steel ball bearing of the same size as the barrel lands through the bore. This flattens out any irregularities in the bore, leaving a mirror-like finish.
Very high-quality weapons and military rifles, in which the bore is subjected to extremely high temperatures, can also have the bore of the barrel chromium-plated. This results in an extremely hard, mirror- like surface which is very resistant to corrosion, metal fouling and bore wear.
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