Recommended Procedures for Documenting Bullet Trajectory
Crime scenes involving the use of firearms present unique challenges for the crime scene investigator, but using relatively simple techniques it is often possible to reconstruct the events that give some indication as to what occurred during the actual discharge of the weapon. By this I mean it is possible to determine the actual path or trajectory of the bullets, and using this information, determine the location of the shooter.
A number of factors must be taken into account including the position of cartridge cases ejected from automatic and semi-automatic weapons. It is therefore essential that the exact position of spent casings be marked and documented before any other investigative procedures are followed.
Document (Photograph & Sketch) Positions of Spent Cartridge Casings
Photography is the first necessity so it is imperative that the Bio Pros be afforded absolute security. Until spent cartridges are properly recorded-all foot traffic must be banned from the area.
Each spent casing should be marked using standard crime scene evidence identifiers such as “evidence tents,” placards, or similar devices.
Typically, overall scene photos are taken first, followed by medium distance and close-ups. Be certain to include other nearby objects in these photos to better establish the true position of each object photographed.
It is also imperative that a crime scene sketch be prepared once photography is completed. Each spent casing should be located on the sketch that includes either triangulation or coordinate methods of measurement.
The physical location of spent casings may tell a unique story of their own. Placement of these objects results from normal ejection by the weapon and may provide limited data as to the location of the shooter, the direction of shots fired and possibly the path taken by the bullet(s). Ejected casings may also substantiate or refute statements from witnesses, victims or suspects.
Methods For Determining Bullet Trajectory
Historically, the technique of “stringing” a crime scene may date back over 70 years. Stringing has been, and continues to be used at crime scenes for the purpose of determining the source of blood spatter and the path of bullets.
Normally speaking, a firearms examiner would be called upon to render expert testimony with regard to the physics and trigonometric calculations regarding bullet travel. But experts with these qualifications are often few in number. It is therefore incumbent upon the crime scene investigator to provide this “expert” with the documentation needed to draw his conclusions.
While other factors may contribute to a determination of bullet trajectory-the most important fact required for there to be even the slightest degree of accuracy is for the bullet(s) to have passed through at least two objects. This will include:
A door and a wall
A two-sided wall
A window and/or a wall, door or victim, etc.
While stringing the crime scene has been the most frequently used method for documenting bullet travel, it does have its short-comings. When a bullet passed through a window, sharp edges are created, therefore if string is used it must be protected (tape or drinking straw). If more than just a few feet of travel is involved, string is susceptible to droop or sagging. It is therefore advisable to use a strong nylon or other synthetic string rather than cotton string.
Alternatives to Using String
An ideal alternative to use are rods of different diameters. With the exception of hollow aluminum rods, most metals like steel, copper or brass may tend to be too heavy. Also consider wooden dowels or plastic and fiberglass rods.
Tools and Equipment Needed for Determining Bullet Trajectory
String (if this is all that is available) Nylon (or equivalent synthetic)
Plastic or fiberglass rods (preferred), multi-colored if possible)
Adhesive tape (duct tape, surgical tape, even fingerprint lifting tape)
Measuring tapes (25-100 ft. w/metric scale)
Photo ID tape or stick-on labels
Evidence collection kit containing sterile swabs and collection containers
Evidence marking labels (adhesive-type)
Protractor (clear plastic) or,
Inclinometer – for measuring angles and slopes
Directional compass (camping-type with aiming sight)
Graph paper for sketching
Carpenter’s or brick mason’s string (bubble) level. This device is only used if an inclinometer is not available.
Crime Scene Investigation Guidelines
Due to the variety and complexity of crime scenes, it is impractical to publish specific guidelines. The following guidelines are offered as a starting or reference point. The actual methods and procedures followed will be dependent upon the experience, judgment and training of the crime scene investigator.