Ballistics 5

Lecture 5

Firearms barrel production

Barrels used for firearms manufacturing are made up of a solid piece of metal

Metal for this purpose is carefully selected for its chemical and metallurgical structure in order to assure high-quality products after several machine operations and heat treatment.

Process of barrel making

by means of “drilling”, a rough hole of uniform diameter is produced on the selected metal thus producing several scars and scratches on its interior surface that becomes permanent

same is produced when a firearm is being rifled

Whatever polishing and finishing efforts of the manufacturer to make its surface clear, said scratches and scars remain permanent and cannot be completely removed

After killing someone, if the barrel is changed, how will we link it to the gun

If you want to change any part of a licensed weapon you have to officially report the change to the government

Process of a bullet leaving the gun

The bullet rests in the chamber, when the hammer hits it, the cartridge case and the bullet projectile separates and goes through the barrel, you will see the hammer marking, rifle markings and barrel markings

Bore diameter

This is the diameter of the barrel without considering the depth of the grooves

Groove diameter

This is the diameter of the barrel also considering the depth of the groove.

The Brunswick style of rifling

used in military muzzleloaders had two deep grooves that were a close fit with a belt on the bullet, but aligning the belt of the bullet with the grooves in the barrel proved to be difficult during battle.

The 4 most common types of rifling

Button rifling

Single point cut rifling

Broach rifling

Hammer forging

Button rifling

is a cold forming process that swages rifling in the barrel with a carbide tool that has grooves in a specific dimension and twist rate.

is a single pass operation, where the button is pushed or pulled at a controlled rate through the bore of a barrel blank, resulting in the desired lands and grooves.

Pros and cons of button rifling

Pros

This cold forming process offers benefits to the finished barrel.

It work hardens the surface layer of the barrel

Increasing durability and barrel life

It also results in an improved finish.

Cons

An additional stress relieve step is required adding cost and time.

The stress relieving operation is used to relieve stress introduced in the bore of the barrel because of the cold forming process.

A uniform outside diameter is necessary due to the swaging performed when button rifling.

A unique tool is needed for every twist.

Single point cut rifling

Also referred to as Hook Rifling, this is a metal cutting process that uses a carbide “hook” to cut a single groove in the barrel.

The process requires several passes to cut the groove to its final depth.

The tool cuts one of the grooves, indexes, then cuts the next groove and indexes again until the desired number of grooves have been cut.

The hook is advanced out or deeper via a wedge-type system, and the process is repeated.

The whole cycle may include many passes to achieve the final depth of the groove in a barrel.

Pros and cons of single point cut rifling

Pros

The metal cutting process does not introduce additional stresses in the metal, so the barrel does not require post stress relieving.

Manufacturers have great control over groove depth due to the minimal depth of cut per pass and the number of passes per cycle. By simply running another pass, the groove can be cut slightly deeper to achieve the desired groove diameter.

Cons

is a very time consuming process due to the number of cuts and minimal amount of material removed per cut.

It does require a skilled tradesman to operate the machine and process parameters as compared to other rifling methods, where variables are highly controlled by the machine.

Bore diameter uniformity before the rifling process is very important as the rifling tool body locates on the bore while cutting the groove.

Broach rifling

is a metal cutting process that uses a long tool with multiple teeth that take small amounts of material out of each groove.

The tool is fed down the barrel with a rotation that matches the twist ground in the tool, with each progressive tooth increasing the depth of cut.

This is a similar process to single point cut rifling, but only requires a single pass. It can be done on a similar machine to that of button rifling

Pros and cons of broach rifling

Pros

Similar to cut rifling, additional stress is not introduced into the material.

Broach rifling does offer a distinct cycle-time advantage, which is more comparable

to that offered by button rifling, by using a single pass.

 

Cons

With broach rifling, a tool is needed for every twist rate, and the tool design prohibits progressive twist.

Surface finish is not as smooth as that resulting from other rifling processes.

Tooling costs can be very high, and the tools are relatively fragile, potentially adding even more costs or interrupting production

Hammer forging

The cold hammer forging process pushes an oversized metal tube over a hardened mandrel, which has the negative of the grooves ground into the outside profile.

With some modern mandrels, the chamber is also included.

During the process, the tube is fed and forged in small sections, and results in a finished barrel blank that is formed not just to the desired twist and caliber, but also length and sometimes, outside contour.

Pros and cons of hammer foraging

Pros

A metal forging process results in increased barrel durability.
Hammer forging also reduces the number of finishing steps, shortening cycle time and lowering manufacturing costs.

once setup and proven, is very repeatable for production barrel manufacturers.

 

Cons

requires a high initial investment for dedicated machinery. Similar to other rifling methods.

A specific mandrel is required for every twist, and progressive twist is not possible.

The parts of a firearm which come in contact with cartridges are firing pin, the breech face, the extractor, the ejector and the chamber.

The bullet comes in contact with the leed & barrel.

These parts are made of steel by cutting, hammering, filing reaming and polishing. The tools used in their manufacture are applied at random.

The cutting surfaces of the tools used, continue to change because of wear and tear they suffer due to their action on steel.

The finished surfaces of the relevant portions of all firearms are, therefore, never alike even when the two surfaces are made with the same set of tools, one after the other.

Rifling mark

they are found on the cylindrical portion of the fired bullets or slugs and are caused by the rifling inside the barrel. These marks are specifically classified as “land marks” and “groove marks”

Skid marks

form on the bearing surface of bullets as they enter the rifling of the barrel before the bullet engages the rifling. Are typically produced by revolvers and have the appearance of widening the land impressions at the nose of the bullet

Shaving marks

Most commonly these marks are found on the bullet fired from a revolver due to poor alignment of the cylinder with the bore

Striation marks

The usually microscopic marking on the surface of fired ammunition components caused by combination of force and motion; these marks can contain class and/or individual characteristics.

Slippage marks

bullet fired from a worn-out barrel, oily barrels and slightly oversized barrel

Breechface production

This is the very important portion of the gun that strikes the cartridge case and the primer is forced backward when a cartridge is fired in the chamber. During the process, breech face marks will be imprinted at the primer and base of the cartridge case that will serve as an important identification for a particular firearm.

Striated action marks

can be produced on cartridge cases by contact with a number of different areas within the firearm. Some of the more common striated action marks include chamber marks; shear marks, firing pin drag marks, extractor marks, and ejector marks.

The chamber marks

Roughness in the chamber of a firearm can scratch the outer walls of a cartridge case when loaded and removed from the chamber. Most chamber marks occur after the cartridge is fired. Cartridge cases expand when fired pressing out against the walls of the chamber. When they are pulled out of the chamber, the sides of the cartridge case can be scratched.

The shear marks

When a cartridge case is forced backward from recoil the primer embeds itself in the firing pin hole. As the slide of the pistol starts to recoil, the barrel will drop slightly as the action opens. The dropping barrel forces the cartridge case to move down slightly and when this happens the lower edge of the embedded primer is sheared downward and out of the firing pin hole

The ejector marks

Ejector marks are sometimes created when cartridges or cartridge cases are ejected from the action of a firearm. Ejector marks can be either striated or impressed

Extractor marks

Another action mark, usually found in striated forms is those created by the extractor of most auto-loading or repeating firearms. The extractor is a small part sometimes resembling a hook that is used to remove a cartridge or cartridge case from the chamber of a firearm.

Magazine Lip Marks

These markings are found near the rim of the cartridge cases and are caused by magazine lips during the loading of the cartridges into the position of firing.

Firing Pin Drag Marks

In a similar process, striated marks called firing pin drag marks can be produced. When the firing pin springs forward to strike the primer of a cartridge, it may remain slightly forward and embedded in the primer.

class characteristics

Bore diameter

Number of lands and grooves

Lands

Grooves

Direction of twist

Width of lands

Width of grooves

Depth of grooves

Pitch of rifling's: it is a measure of the twisting of the land and grooves. It refers to the distance advance by rifling's in one complete run (360 degrees)

Cartridge Caliber

Shape of firing pin

individual characteristics

They serve to identify a particular gun. These individual characteristics are generally found on the interior – the surface of a gun barrel, which registers into the fired bullet while inside the bore of the gun and to the base of the cartridge case when it exploded inside the chamber of the gun.

Toolmark examination

A tool is defined as the harder of two objects which, when brought into contact with each other, results in the softer object receiving a toolmark.

Tools (e.g., screwdrivers, firearms, bolt cutters, etc.) will bear unique microscopic characteristics due to the manufacturing processes they undergo and use and abuse.

These characteristics will mark surfaces (e.g., locks, cut wires, fired bullets, etc.) with class and individual characteristics.

These class and individual characteristics are reproducible and identifiable with a particular tool.

Principles of cartridge examination

The breech face and firing pin of every single firearm have individual microscopic individualities of their own;

Every firearm leaves its fingerprint or thumbprint on every cartridge it fires;

That, since the breech face of every weapon, has the individual distinction, the imprints of all cartridge cases fired from the same weapon are the same.

Principles of bullet examination

No two barrels are microscopically alike or identical as the internal surface of the bore or barrels all possess individual characteristics of their own;

When a bullet is fired from a rifled gun barrel, rifling's are engraved therein, which vary in its minute details with other firearms, even of the same type.

Every barrel leaves its mark known as a “thumb mark or fingerprint” on every bullet that passes through it.

Instruments used for analysis

Bullet comparison microscope

Stereomicroscopes

Onoscope

Shadowgraph

Caliper

Helixometer / micrometer

Analytical / torsion balance

Bullet recovery box

Magnifying glass

Taper gauge

Electric marker