Topic 7 - Internal Ballistics

Question 1

How can ballistic efficiency be measured?

Ratio

Answer 1

Ballistic efficiency can be measured as a ratio of peak chamber pressure to projectile velocity.

Question 2

What are the main stages of the internal firing process?

Answer 2

• Lock time
• Ignition time
• Barrel time

Question 3

What is the lock time?

Activation

Answer 3

The ‘lock time’ is the time between the activation of the firing mechanism and ignition of the primer charge.

Question 4

What is lock time mainly dependent on?

Trigger mechanism

Answer 4

This is mainly dependant on the design of the trigger mechanism, which links the trigger to the firing pin assembly.

Question 5

What is the trigger mechanism composed of?

Spring pressure

Answer 5

It consists of a system of levers, known as sears, which hold the firing pin or hammer under spring pressure until the trigger is operated.

Question 6

What happens after the firearm is cocked?

Compressed, sear, activate primer

Answer 6

• When the firearm is ‘cocked’, the firing pin spring is compressed and then mechanically held by a sear.
• The energy stored within the spring is then released when the trigger is pulled and is used to activate the primer.

Question 7

What mechanism are most modern firearms required to have?

Rough handling

Answer 7

Most modern firearms are required to have a ‘drop safety’ mechanism, preventing the weapon from discharging if subjected to rough handling or if dropped.

Question 8

What is trigger safety?

Free to operate

Answer 8

• This is a simple mechanism that physically blocks the external trigger action.
• However, the rest of the firing mechanism (particularly the sears) is free to operate independently of the trigger.
• Normally activated by a small button or lever located behind the trigger.

Question 9

What is sear safety?

Tampering

Answer 9

• A mechanical safety mechanism, that blocks the action of one or more of the trigger sears.
• This is the type of safety most commonly damaged by amateurs, when tampering with the trigger mechanism.
• This category includes the grip safety and fire selection levers

Question 10

What is a sear?

Sliding contact surface

Answer 10

• A sear is any mechanical part of a trigger mechanism that has a sliding contact surface.
• Most modern triggers have multiple sears.

Question 11

What options are possible with a fire selection lever?

Answer 11

Some firearms have a ‘multi-purpose’ control from which the operator may be able to select:
o Safe
o Single fire (or semi-automatic fire)
o Burst fire
o Automatic fire

Question 12

What is the firing pin safety?

Independant

Answer 12

• This is a mechanical system that blocks the action of the firing pin.
• It is the most reliable and effective form of safety catch as it is unaffected by operation of the trigger or trigger sears.
• It can be independent of the trigger system, usually built into the bolt or slide so very hard to tamper with

Question 13

What is a magazine interlock?

Answer 13

A magazine interlock is an internal mechanism that engages a mechanical safety when the firearm’s magazine is removed.

Question 14

What is a magazine interlock designed to do?

Answer 14

• It is designed to prevent accidental discharge where the magazine has been removed but a round is still chambered.

Question 15

How do you perform a safety failure test?

Low velocity impacts

Answer 15

1. Ensure the unit is unloaded.
2. Cock the action and dry fire it repeatedly.
3. Then re-cock the action and subject the unit as a whole to low-velocity impacts in all 3 planes (back, side, top).
4. Operate the trigger with the safety engaged.
5. Operate the firearm in the same way as when the accident occurred.
6. Repeat the process 10 times noting the result of each test.
   * Any single failure will support the claimed accidental discharge.
   * By its nature this is a subjective test (and also potentially destructive to the firearm) and as such will carry a lower weighting as evidence.

Question 16

How do you perform a trigger test and what is the purpose?

Solid & safe

Answer 16

• The unloaded weapon must be secured in a solid and safe position, the action cocked and then a force meter attached to the trigger and the trigger operated.
• The force required to activate the firing mechanism should be recorded for each test.
• This test should be repeated 10 times and any extreme drop in the force required to activate the trigger may support an accidental discharge claim OR that the firearm has been tampered with.

Question 17

What is ignition time?

Answer 17

The ignition time is the time from activation of the primer charge to initial movement of the projectile.

Question 18

What does ignition time depend on?

Type

Answer 18

• Primer type
• Propellant design
• Chamber dimensions
• The ‘tightness’ of the bullet in the cartridge neck

Question 19

What happens when the firing pin contacts the primer?

Deforms. crushes

Answer 19

• When the firing pin contacts the primer it deforms the soft primer cap crushing the primer mixture against an internal anvil.
• The primer mixture is a primary explosive (e.g. lead styphnate) and this impact causes it to detonate.
• The resultant hot gases pass through the flash hole and ignite the main propellant charge in the cartridge case

Question 20

What is burn rate determined by?

Size, type, chemistry

Answer 20

• Burn rate is determined by grain size and shape, primer type, propellant chemistry and chamber dimensions.
• The burn rate is chosen carefully for each individual application.

Question 21

What happens when the propellant is ignited?

Ignited by primer flash

Answer 21

When the propellant is ignited by the primer ‘flash’, a large amount of gas is produced and the pressure in the cartridge rises rapidly until it is sufficient to unseat the projectile from the cartridge neck as it begins to move.

Question 22

What is barrel time?

Answer 22

• The ‘barrel time’ is the time from the first movement of the projectile to when the projectile exits the muzzle.

Question 23

What does barrel time depend on?

Friction, chamber

Answer 23

• Projectile shape, size (calibre), and mass
• Internal barrel friction
• Rifling type and rate of twist
• Chamber pressure and propellant burn rate

Question 24

What is different about the internal ballistics in spring-operated air weapons?

Compressed spring, drives projectile

Answer 24

• The trigger releases a compressed spring instead of activating a primer
• The spring pushes a piston which compresses air into the barrel whiches drives the projectile down the barrel time
• Compression time instead of ignition time

Question 25

What is different about the internal ballistics in pre-charged pneumatic air weapons?

Fixed amount of compressed air

Answer 25

• A valve system, activated by the trigger, allows a fixed amount of compressed air out of the storage cylinder and into the barrel, driving the projectile.
• Valve operation time instead of ignition time

Question 26

How do you test muzzle energy?

Muzzle energy = EK, maximum charge

Answer 26

• The weapon should be charged to maximum if it has a pre-charged pneumatic mechanism.
• 3 different masses of pellet must be used:
  
  • Heavy
  • Medium
  • Light
• 10 of each pellet type are fired from across a
  chronograph, which measures the projectile’s
  velocity.
• All results should be recorded contemporaneously in the form of a S9 statement
• Use Ek = 1/2 mv^2 to determine muzzle energy
• The results must not be averaged if any single projectile delivers a muzzle energy which exceeds the energy limit then the weapon is deemed to come under the 1968 Firearms Act.

Question 27

What can a poorly considered load lead to?

Drilled out

Answer 27

• The chamber pressure exceeds the design/proof limits.
  
  • In this case, the firearm can explode.
• Low ignition pressure.
  
  • In this case, it can cause bullet jams in the barrel. This may not be apparent to the firearm operator until it’s too late.
  • The bullet must be drilled out.

Question 28

What safety method is 2nd best for preventing accidental triggers?

Closer

Answer 28

• Sear safety
• This is more reliable then the trigger safety since it’s closer to the firing pin, but failure will still allow the firing pin to move independently.
• This is the type of safety most commonly

Question 29

What happens in barrel time?

Answer 29

During this time, increased chamber and barrel pressure, which continues to accelerate the projectile, is maintained by the propellant continuing to burn until the projectile leaves the muzzle.