Topic 2 - Barrels, rifling and twist

Question 1

what components are in the cartirdge that goes in the chamber and barrel of a gun

Answer 1

primer
cartridge case
bullet

Question 2

what is the firing process inside the chamber/barrel?

Answer 2

Firing pin –> chamber –> barrel

1) primer activation by impact from the firing pin
2) propellant starts to burn due to heat coming through flash hole - gas and pressure produced - cartridge expands/obturates
3) pressure becomes sufficient to unseat bullet from the cartridge, launching it down the barrel as the propellant continues to burn.

Question 3

what is rifling?

Answer 3

most bullets behave like mini gyroscopes in the air, having been spun down the barrel of the firearm by engaging with rifling

rifling is the spiral grooves that are formed in the barrel wall

the spin rate is typically in the region of 250,000 rpm (rotations per minute)

spin imparts accuracy and in flight stability

Question 4

what does rifling look like?

Answer 4

Land - the raised area (HILL)

Groove - the sunken area (VALLEY)

Question 5

what are lands, landmarks, grooves, groovemarks and caliber (using the barrel)?

Answer 5

• Lands: Raised section of barrel wall (hills).
• Land marks: Sunken areas on the surface of a fired bullet.
• Grooves: Sunken areas between lands in the barrel wall
  (valleys).
• Groove marks: Raised areas on the surface of a fired
  bullet.
• Caliber (using the barrel): Measured between 2 grooves
  opposite one another in the barrel.

Question 6

what is twist direction?

Answer 6

• Twist direction: rifling can be cut in either left or right hand
  configuration. If you were to look down the barrel (which
  you should never do), you will see the rifling as either:
• Clockwise: Right hand
  or
• Anti-clockwise: Left hand.

Question 7

what is twist rate?

Answer 7

• Twist rate: classically expressed in terms of the number of
  revolutions per inch of barrel length.
• This ratio is commonly expressed by designations such as 1:10,
  1/10 or 1 in 10 twist, the 1 represents 1 twist, the 10 represents
  inches of barrel length. So, a 1 in 10 twist is, 1 complete bullet
  revolution in every 10 inches of barrel length travelled through.

Question 8

what is a described angle?

Answer 8

• Described angle (θ): Angle between the longitudinal bullet axis
  and inscribed rifling marks (usually less than 10º).

Question 9

how do you examine rifling marks?

Answer 9

1. Width of land marks: vernier calliper or microscope.
   * Feel the surface of the bullet for the regions that are sunken.
2. Width of groove marks: vernier calliper or microscope.
   * Feel the surface of the bullet for the regions that stick out.
3. Number of lands: 1 eyeball.
   * Just count them, remembering where you started!
4. Described angle: digital microscope.
   * Photograph and measure on images with protractor, or use photo
   software.

Question 10

what do you measure for the rifling marks?

Answer 10

width of each indented area gives the land mark width

each of the areas between the stripes is a rifling mark

width of each protruding area gives the groove mark width

the marks are not the lines but the area between the lines.

Question 11

how do you calculate the twist rate of the rifling?

Answer 11

C = bore circumference = PI (TT) x diameter (D)

L = length of the barrel for one twist

θ = described angle

L = C / tanθ

or

L = TT x D / tan θ

Question 12

what is the conversion of metres to inches?

Answer 12

1 metre = 39.37 inches

Question 13

what are the two methods of producing standard rectangular rifling?

Answer 13

• Broach-cut: Uses a single point tool to cut one groove at a time, with each groove taking several cuts to each the desired depth.
• Electro-chemical etching: Uses an electrolyte, which is
  passed through grooves in a tool whilst in contact with the inside of the barrel to etch away the material and form the barrel grooves.

Question 14

what are the two methods of producing polygonal rifling?

Answer 14

In polygonal rifling, the traditional sharp lands and grooves are replaced
with “hills and valleys”, usually as a hexagon or octagon. This is now
becoming the industry standard and is typically produced by:

• Hammer-forged:
• The hardest wearing rifling type giving a very precise and clearly defined pattern and generally used in military weapons.
• A tungsten carbide ‘mandrel’ with the rifling profile machined into it is inserted into an oversized barrel, which is then hammer forged onto it to form the barrel grooves.
• Button-formed:
• This process produces a precise polygonal pattern.
• It is created by a tungsten carbide ‘button’ with the rifling profile machined into it being forced through the barrel to form the grooves.