There are so many psychological aspects to the 7/2/13 repeat NCIS episode titled "Recovery": why is it so imperative for investigators to have psychological evaluations?; how detrimental are romantic relationships with co-workers?; what are the ramifications of adoption?; and how do personal issues influence stress and sleep deprivation?.
However, let's stick with physical evidence - the bullet found in the front seat of the vehicle. The mold spores helped to establish that the murder weapon was an older one. The striations on the bullet were not really stressed in this episode but Abby did use the number of lands and grooves and the twist to determine the possibilities of the weapon type used.
When a weapon is made the barrel is drilled and a particular number of depressions are created and in either a right hand or left hand twist. The number of "lands and grooves" along with the twist help the investigator in narrowing down how manufacturers bore their particular barrels and therefore a list of potential brands of weapons are made available.
The markings created when the barrel is tooled create the striations on the soft lead of the bullet as it travels through the barrel once the gun is fired. But first one needs the suspected weapon to actually make the comparison between the striations on the bullet from the scene or the body and that of a bullet fired from the suspected weapon.
This is the one time that it's good to have 'an interesting new twist' in the case investigation.
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