It's a sad fact of police life that investigators and technical specialists are often not available for response to "routine" or "mundane" incidents. Shrinking budgets, reallocations of resources, and growing rates of more serious offenses far too often relegate shootings without injuries to, at best, follow-up investigations, if any investigation at all. However, all is not lost; the patrol officer, armed with the knowledge of shooting reconstruction, can often piece together enough facts to allow follow-up from the patrol level, possibly taking a shooter off the street and making the beat a safer place for all.
The value of certain evidence is obvious. Shell casings, shot shell hulls, and projectiles are all capable of telling the firearms examiner a great deal -- the type of firearm, possibly the specific make and model of firearm, and with standards available, identifying the specific firearm. But it may also provide the officer on the street valuable information.
First, as with any evidence, document. If you have a camera, photograph the scene. If there are multiple shots, and brass lying in apparent groupings, document them, measure the rough diameter of the pattern, and if there are shoe impressions to relate to firing position, document and measure their relationship to the brass.
When collecting the evidence, remember that fired empty cases may be processed for latent fingerprints or transferred DNA. Use gloves to pick them up, and place them immediately into evidence bags.
If there are two or more holes on the scene that can be attributed to one bullet, you can track a trajectory. Various methods are in use: lasers, string, etc. Generally, string or yarn is cheapest and easiest to work with. Merely stretch the string from one hole to the other, keeping it under some stress to keep it straight. Now you can stretch the string in either direction, being careful to keep it straight. It will give you a path of flight, may locate an impact area, and should bring you into the area where the shooter was located. Multiple traceable shots further aid in reconstructing the events that occurred during the shooting, allowing one to determine shooter movements or changes in aim during the event.
Glass that has been impacted by a bullet or bullets will tell much. Basic window glass, when impacted by a projectile, will flex as it is penetrated. As a result, the entry will be roughly the size of the projectile, but the exit will be cratered, expanding in diameter away from the entry. We now know the direction of impact.
The impact will also result in two types of fractures to the glass. The sunburst-appearing rays around a penetration in glass are referred to as radial fractures; the circular fractures that complete the spider web effect are known as concentric fractures. Of most value is that the radial fractures of subsequent shots will not cross the fractures of the preceding shots. Thus, it becomes possible to determine which shot was first, second, etc.
Tempered glass, such as side windows in autos and sliding glass doors, is designed to shatter in a myriad of small pieces, to lessen injury potential. However, if it is laminated to tint material, it will often stay intact and allow one to observe the direction of impact. As with window glass, it will crater in the direction away from impact.
A "ricochet" will also indicate direction of the shot. The impact will begin with a long, thin entry mark, and a shallow impact which will then grow deeper into the surface of the material. Especially on sheet metal surfaces, the projectile may completely penetrate the surface, again showing by its depth that the bullet impacted at the other end and dug itself deeper.
A semi-automatic firearm will eject fired cases or hulls in a fairly reproducible pattern. This pattern will be conditioned on several factors --the specific firearm, the specific ammunition, and also the style of the shooter.
Every firearm has little differences from its mates when manufactured. These nuances will demonstrate themselves in the pattern--slight differences but nonetheless differing. Similarly, ammunition will affect the pattern. Velocity will also influence the pattern--differing velocities and Newtonian physics will result in altered patterns. Finally, the shooter's style--a Weaver hold as opposed to the infamous gangbanger style, body type, a slight or weak-wristed shooter as opposed to a muscular, strong-wristed shooter--will affect the pattern that will be demonstrated on the scene. If evidence identifies a specific firearm, knowledge of its basic pattern deposition will allow the officer to infer about the shooter's abilities.
So far, this sounds like useless knowledge. Indeed, it has little if any court value. But from an intelligence standpoint, it may be gold. That is the field of the street cop--intelligence gathering and interpretation. Much like recognizing a burglar by M.O., recognizing a brass pattern as being from a particular shooter aids in knowing how many shooters are active in the neighborhood.
Be aware also of the ammunition being used. As semi-autos will leave behind brass, when you collect it, take notice of it. Is it brass, steel, nickel plated, or another material? What does the headstamp say? The headstamp, the stamped on information on the base of the shell casing, may tell the manufacturer by name, or identify the manufacturer by a code, as with much of the imported Russian and Chinese 7.62x39, 7.62x25, and other military ammunition often obtained cheaply. It may also identify the specific caliber, the date of manufacture, etc. Take note of this information -- how many shootings does the same ammo show up at? Has a new headstamp suddenly shown up? Check your local gun stores for stock--look at their headstamps, and see if this may have been locally obtained. Perhaps the store personnel can identify specific customers, or may have security videos that may shed light on the purchasers.
Evidence is not only the province of investigators. Aggressive police patrol work can uncover and interpret evidence, use it to solve individual crimes, or gather it as intelligence that may help spot a trend or tie multiple offenses together. With investigative resources often being stretched thin, it is beneficial for a patrol officer to take steps to recognize, gather, and understand evidence, especially when it pertains to "minor" shooting incidents that may lead to more tragic consequences if not intercepted early.