Topic 2 - Barrels, Rifling & Twist

Question 1

What does the chamber hold?

Answer 1

The cartridge case

Question 2

What is the general firing process?

Answer 2

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

Question 3

What is rifling?

Answer 3

Rifling is the spiral grooves that are formed inside the barrel wall.

Question 4

What is the typical spin rate?

Answer 4

250,000 rpm

Question 5

What does the spin do?

Answer 5

The spin imparts accuracy and in-flight stability to the bullet.

Question 6

What are lands?

Answer 6

Raised section of the barrel wall (hills)

Question 7

What are land marks?

Answer 7

Sunken areas on the surface of a fired bullet

Question 8

What are grooves?

Answer 8

Sunken areas between the lands in the barrel wall (valleys)

Question 9

What are groove marks?

Answer 9

Raised areas on the surface of a fired bullet.

Question 10

How can you measure the caliber using the barrel?

Answer 10

Measure between two grooves opposite one another in the barrel.

Question 11

What is twist direction?

Answer 11

• Rifling can be cut in either left or right-hand
  configuration. If you were to look down the barrel you will see the rifling as either:
• Clockwise: Right hand
• Anti-clockwise: Left hand.

Question 12

What is twist rate?

Answer 12

Classically expressed in terms of the number of
revolutions per inch of barrel length.

Question 13

What is the described angle?

Answer 13

Its the angle between the longitudinal bullet axis and the inscribed rifling marks and is usually less than 10.

Question 14

What type of characteristic are rifling marks?

Answer 14

Class characteristic

Question 15

How do you examine rifling marks?

Answer 15

• Width of land marks
• Width of groove marks
• Number of lands
• Described angle

Question 16

How do you calculate bore circumference?

Answer 16

C = Bore circumference
C = Pi x Diameter

Question 17

How do you calculate the twist rate of the rifling?

Answer 17

L = Length of the barrel for one twist
θ = Described angle
L = C / tan(θ)
L = Pi x D / tan θ

Question 18

What are the different rifling profiles?

Answer 18

• Standard
• Polygonal
• Trapezoidal

Question 19

How are standard rifling marks produced?

Answer 19

• Broach-cut: Uses a single-point tool to cut one groove at a time, with each groove taking several cuts to 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 20

What is the profile for standard rifling?

Answer 20

The standard (rectangular) profile of rifling shows sharp, angular lands and grooves that are well-defined.

Question 21

What is the profile for polygonal rifling?

Answer 21

• 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.

Question 22

How are polygonal rifling marks produced?

Answer 22

• Hammer-forged: The hardest-wearing rifling type gives a very precise and clearly defined pattern and is 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.