Barrelling and twist rate Flashcards
(20 cards)
What is rifling?
Spiral grooves formed in the barrel wall
Rifling allows bullets to spin, enhancing accuracy and stability in flight.
What is the typical spin rate of bullets during firing?
Approximately 250,000 rpm
This high spin rate helps stabilize the bullet in flight.
What are lands in the context of rifling?
Raised sections of barrel wall (hills)
Lands are crucial for engaging the bullet and imparting spin.
What are grooves in rifling?
Sunken areas between lands in the barrel wall (valleys)
Grooves help in the stabilization of the bullet as it travels down the barrel.
Define caliber in relation to a firearm barrel.
Measured between 2 grooves opposite one another in the barrel
Caliber indicates the diameter of the bullet and the barrel.
What is twist direction in rifling?
Can be cut in either left or right hand configuration
Right-hand twist is clockwise, left-hand twist is anti-clockwise.
What does twist rate indicate?
Number of revolutions per inch of barrel length
Expressed as ratios like 1:10, indicating how many twists per inch.
What is the described angle (θ) in rifling?
Angle between the longitudinal bullet axis and inscribed rifling marks
Usually less than 10º.
What are class characteristics in bullet examination?
Features that narrow down possible firearm types but not specific firearms
Includes width of land marks, groove marks, and the number of lands.
What is the bore circumference formula?
C = π x diameter (D)
This is used to calculate the twist rate based on the bullet’s caliber.
How is the twist rate calculated?
L = C/tan(θ)
or
L = 𝜋D/tan(θ)
(Length of barrel for one twist = Bore circumference/tan (described angle))
The calculations involve converting measurements into appropriate units.
What is the significance of accuracy in rifling measurements?
Care must be taken due to limitations/errors in measurements
It is common to round twist rates to the nearest inch in the industry.
What is standard rifling?
Profile with sharp, angular lands and grooves
This type of rifling is well-defined and traditionally used.
Describe broach-cut rifling.
Uses a single point tool to cut one groove at a time
Each groove takes several cuts to achieve the desired depth.
What is polygonal rifling?
Replaces sharp lands and grooves with ‘hills and valleys’
Typically produced by modern methods and is becoming the industry standard.
What is hammer-forged rifling?
Hardest wearing rifling type giving a precise pattern
Commonly used in military weapons for durability.
What is button-formed rifling?
Creates a precise polygonal pattern using a tungsten carbide button
The button forces through the barrel to form grooves.
Fill in the blank: The angle between the longitudinal bullet axis and inscribed rifling marks is called ______.
described angle (θ)
This angle is important for understanding the rifling characteristics.
what stability does twist supply the bullet?
twist counteracts the aerodynamic forces that would cause the bullet to tumble. the higher the twist rate, the more stable the bullet is during flight.
How do you calculate twist rate using velocity, bullet diameter and bullet length?
Greenhill formula: Twist=CD^2/L
, where C is 150 for velocities around 2800 ft/s and 180 for velocities above 2800 ft/s, D is the bullet diameter in inches, and L is the bullet length in inches.
