Matching Techniques for Barrels & Bullet Examination

Button/Swaging Rifling

Swaging technique that uses a plug or button made of extremely hard material bearing a negative impression of the rifling. - Plug is hydraulically pressed through the bore of the barrel, stretching the inside diameter and transmitting the rifling characteristics. - Button is slightly larger than bore diameter. - Cold working the metal causes it to be harder but with more stress on the metallic structure - Forms the metal and presses into it, doesn't cut

Base diameter

(image) - Measure from groove to groove aka the raised edges - This may not be measurable from the damage to the bullet, which is why we use the weight to find caliber

Parts of a bullet design

(image) - Ogive: entire curve of the bullet - Nose: the tip - Base - Bearing surface: straight and widest part of the bullet that comes into contact with the rifling of barrel. aka the caliber - Meplat: shape of the flat or open tip that determines how the bullet moves through the air. not seen on round noses

Bullet type

(image) - Round nose is the most common - Wad cutter seen for target practice and not common at crime scenes - Jacketed bullets have the best rifling from the harder metal - Jacketed hollow points expand when in contact with the target

Jacket type

(image) Yellow = Brass Silver = Nickel Orange = Copper Full Metal Jacket = metal jacket over the bullet but it has an open base Total Metal Jacket = metal jacket covers the whole bullet, including the base

Base description of bullet

- Open or jacketed - Flat or concave

Scrape/Hook Rifling

- Scraping type cutter - a rod slightly smaller than bore diameter with a set of one or two hardened steel scrapers. - During each pass of the cutter, some metal is removed. > At the top if each stroke, a small wedge is driven in > Raises the cutting surface enough to continuously remove metal

Bullet Examination Characteristics (9)

1. Bullet type 2. Jacket type 3. Base diameter 4. Weight 5. Number of lands and grooves 6. Direction of twist (left or right) 7. Type of rifling (conventional or polygonal) 8. Land and groove diameter 9. Caliber determination

Conventional (Standard) vs. Polygonal cuts

1. Cut - conventional - sharp well-defined shoulders of groove impression 2. Polygonal - smooth rounded shoulders of the groove impression - seen in glocks (attached image)

Manufacturing Barrels - Types of Rifling (5)

1. Scrape/Hook 2. Broach Cutting 3. Button/Swaging 4. Hammer Forging 5. Electrochemical Rifling

Machining Techniques to make bullets (8)

1. Turning 2. Boring 3. Reaming 4. End Milling 5. Broaching 6. Filing 7. Swaging 8. Electrochemical Machining (ECM)

Hammer Forging Rifling

Also a swaging technique 1. Bored barrel blank is placed over a short mandrel (shaft that spins) that bears the negative of the rifling impress (just like a button). 2. Barrel blank advances over the mandrel and hammers pound the outside of the barrel and impart the rifling impressions to the interior of the barrel. - First attempts often used a larger mandrel but got stuck inside the barrel after rifling. - Shorter mandrels were used because they were easier to manipulate. - The profile of hammer forged barrels is polygonal rifling. > the curved sides of the grooves shoulders allowed the mandrel to be more easily removed. - Forms the metal and presses into it, doesn't cut

Broach Cutting Rifling

Consists of a rod bearing 25 to 30 hardened steel rings each being slightly larger than the one before. - steel rings have notches cut into them that correspond to the dimensions of the rifling - Cuts the grooves into the barrel in one pass - Disadvantage - take more skill to manufacture and the cutting edges are susceptible to chipping thus causing a defect in the rifling - Advantages - make many more barrels in the same time it would using the hook method and there is less wear to the tool because there are more cutting edges

Number of Lands and Grooves

Count the number of land and groove impressions seen - Most common is 6

"Land-Grooveian" Theorem

D = #(L + G)/pi - L = diameter of lands - G = diameter of grooves - # = number of lands and grooves - D = diameter

Land impressions

Depressed helical grooves in a bullet left by the lands inside the barrel from which it fired - use these to ID the bullet because this part of the barrel was created by the machinery

Caliber determination

Determined from the diameter and weights (image of table)

Electrochemical Machining (ECM)

Dissolving metal into a desired shape. - Brass is used as a negative terminal and the work piece as the positive. - The work piece is submerged into a solution of electrolytic salts and electrical current is passed through the solution. - The work piece will dissolve in areas touching the brass - Very effective means of machining by limiting tool wear

Manufacturing Barrels - Honing

Done after reaming is the surface is still rough - Smoothes out any rough areas

Direction of Twist

Either left or right - on the bullet, examine it from the base

Filing Machining Technique

Finishing technique - Used to deburr (smooth) freshly cut edges. - Files are usually hand worked and shaped into to easily fit into small areas. - The cutting edge has teeth that are oriented so that they cut when you push on the object

Manufacturing Techniques for the Barrel (7)

First: Bar/blank stock (raw purified metal) created from turning and refined into a shape for the machining process Then: 1. Drilling 2. Sizing 3. Honing 4. Rifling 5. Lapping 6. Profiling the Barrel 7. Crowning the Barrel

Cannelures

Grooves around a bullet that provide a means of crimping the cartridge case to the bullet - Found on the lands and grooves - Smooth or knurled - manufactured, either a straight line or a line with ridges - Crimping - not an actual cannelure, it is a discontinued line from firing (image)

Manufacturing Barrels - Lapping

Lead Lapping - Comes after the rifling process - Uses lead plugs impregnated with a lapping compound to smooth out the interior of the bore.

End Milling Machining Technique

Machining process - Utilizes a rotating cutter that is perpendicular to the metal work piece. The edges of the cutter have carbide teeth (image) that rip away at the metal. - Forms slots and can be used to face the outside edges of the work piece.

Boring Machining Technique

Machining process - Utilizes a stationary cutting tool and a rotating work piece - Similar to turning, only it cuts the inside - Used to enlarge drill holes from the inside of the work piece. - Considered an internal cutting process.

Broaching Machining Technique

Machining technique - Utilizes a series of cutters that start and finish cutting in a single pass. - Cutting surfaces are arranged so that each cutter is slightly larger than the previous. - Very efficient process and can be used to make a variety of parts

Manufacturing Firearms

Marks left behind from the tools used during the manufacturing of firearm parts allows for the comparison and identification of ammunition components (bullets and cartridge cases) - Every barrel is still unique because the tools slightly change with every barrel - Rifling creates speed and accuracy of the bullet

Land and Groove diameter

Measured on a comparison microscope - Groove impression is the raised edge - it is the indented one in the barrel - Land impression is the indented one - it is the raised one in the barrel

Swaging Machining Technique

Metal working process - Uses dies to create the desired shape. - Type of forging process because it applies compressive force to the metal workpiece - Doesn't cut like the other techniques, it forms the metal

Reaming Machining Technique

Metal working process - Used for smoothing out a drill hole and sizing them to the final dimension

Turning Machining Technique

Performed on a lathe (spinning machine) - Only shapes the outside of the barrel - Workpiece is rotating while a cutter is moved back and forth horizontally. - It is considered an external cutting process

Manufacturing Barrels - Drilling

Process in which a tool creates a hole in the stock - Leaves parallel marks running perpendicular to long axis of the hole - Uses a single flute gun drill (attached photos) - Leaves circular, perpendicular marks inside of the barrel

Manufacturing Barrels - Sizing the bore

Reaming & broaching - Reaming increases the inside diameter of the barrel to the proper caliber - Manufacturers can broach the inside diameter

Manufacturing Barrels - Crowning the Barrel

Recessing the rifling at the muzzle

Manufacturing Barrels - Profiling the Barrel

Turn the outside diameter of rifled bar stock to the proper outside dimensions - Doing this after rifling is important because you want to have the most amount of material present when rifling. - It is then turned on center - meaning the drill hole is now centered with in the profile of the barrel.

Electrochemical Rifling

Uses electricity and the flow of current to etch the impression of the rifling into the barrel

Weight

Weight the bullet itself - Different calibers have ranges of weights - Measured in grains - Listed on ammunition boxes