Lecture 2 - Twist rates and whatnot

Question 1

Firing process

Answer 1

Primer activation by impact from the firing pin

Propellant starts to burn due to heat coming through flash hole - gas and pressure produced - cartridge expands (obturates)

Pressure becomes sufficient to unseat bullet from the cartridge , launching it down the barrel as the propellant continues to burn

Question 2

Rifling

Answer 2

Rifling is the spiral grooves that are formed in the barrel wall.
Spin impacts accuracy and in-flight stability

Rifling has:
Lands - raised area
Grooves - Sunken areas

Question 3

Lands

Answer 3

Raised section of barrel wall

Question 4

Land marks

Answer 4

Sunken areas on the surface of a fired bullet

Question 5

Grooves

Answer 5

Sunken areas between lands in the barrel wall (valleys)

Question 6

Groove marks

Answer 6

Raised areas on the surface of a fired bullet

Question 7

Caliber

Answer 7

Measured between 2 grooves opposite one another in the barrel

Question 8

Twist direction

Answer 8

Rifling can be either clockwise ( right handed )
or anti-clockwise (left handed)

Question 9

Twist rate

Answer 9

Expressed in terms of the number of revolutions per inch of barrel length
Expressed in ratios = 1:10 where 1 represents the twist and 10 represents inches of barrel length - (doesn’t tell you the actual length of the firearm barrel)

Question 10

Described angle

Answer 10

Angles between the longitudinal bullet axis and inscribed rifling marks (usually less than 10’)

Question 11

Examining rifle marks

Answer 11

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

Question 12

Twist rate calculation

Answer 12

L = C/tan(0)

C is the circumference = pie * diameter to get circumference
0(theta) = the described angle
Caliber is the same as the diameter

Question 13

Unit conversion

Answer 13

mm / 1000 = SI units
E.g. 9mm = 9*10-3m

Always convert

Question 14

Inches conversion

Answer 14

Multiply by 39.37 to give inches
Indicates the length of the barrel

E.g. 0.359 * 39.37 = 14.144
1:14 twist rate

Question 15

Standard rifling

Answer 15

Shows sharp, angular lands and grooves that are well-defined

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 toll whilst in contact with the inside of the barrel to etch away the material and form the barrel grooves

Question 16

Polygonal rifling

Answer 16

traditional lands and grooves are replaced by hills and valleys = usually as a hexagon or octagon, produced by :
Hammer-forged:
Gives a very precise and clearly defined patter and generally used in military weapons. A mandrel with the rifling profile attached to it is inserted into an oversized barrel, which is then hammer forged onto it to form the barrel grooves

Button formed:
Makes a precise polygonal pattern
Created by a button with the rifling profile attached into it being forces through the barrel to form the grooves