Topic 7 - Internal Ballistics Flashcards
How can ballistic efficiency be measured?
Ratio
Ballistic efficiency can be measured as a ratio of peak chamber pressure to projectile velocity.
What are the main stages of the internal firing process?
- Lock time
- Ignition time
- Barrel time
What is the lock time?
Activation
The ‘lock time’ is the time between the activation of the firing mechanism and ignition of the primer charge.
What is lock time mainly dependent on?
Trigger mechanism
This is mainly dependant on the design of the trigger mechanism, which links the trigger to the firing pin assembly.
What is the trigger mechanism composed of?
Spring pressure
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.
What happens after the firearm is cocked?
Compressed, sear, activate primer
- 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.
What mechanism are most modern firearms required to have?
Rough handling
Most modern firearms are required to have a ‘drop safety’ mechanism, preventing the weapon from discharging if subjected to rough handling or if dropped.
What is trigger safety?
Free to operate
- 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.
What is sear safety?
Tampering
- 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
What is a sear?
Sliding contact surface
- A sear is any mechanical part of a trigger mechanism that has a sliding contact surface.
- Most modern triggers have multiple sears.
What options are possible with a fire selection lever?
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
What is the firing pin safety?
Independant
- 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
What is a magazine interlock?
A magazine interlock is an internal mechanism that engages a mechanical safety when the firearm’s magazine is removed.
What is a magazine interlock designed to do?
- It is designed to prevent accidental discharge where the magazine has been removed but a round is still chambered.
How do you perform a safety failure test?
Low velocity impacts
- Ensure the unit is unloaded.
- Cock the action and dry fire it repeatedly.
- Then re-cock the action and subject the unit as a whole to low-velocity impacts in all 3 planes (back, side, top).
- Operate the trigger with the safety engaged.
- Operate the firearm in the same way as when the accident occurred.
- 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.
How do you perform a trigger test and what is the purpose?
Solid & safe
- 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.
What is ignition time?
The ignition time is the time from activation of the primer charge to initial movement of the projectile.
What does ignition time depend on?
Type
- Primer type
- Propellant design
- Chamber dimensions
- The ‘tightness’ of the bullet in the cartridge neck
What happens when the firing pin contacts the primer?
Deforms. crushes
- 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
What is burn rate determined by?
Size, type, chemistry
- 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.
What happens when the propellant is ignited?
Ignited by primer flash
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.
What is barrel time?
- The ‘barrel time’ is the time from the first movement of the projectile to when the projectile exits the muzzle.
What does barrel time depend on?
Friction, chamber
- Projectile shape, size (calibre), and mass
- Internal barrel friction
- Rifling type and rate of twist
- Chamber pressure and propellant burn rate
What is different about the internal ballistics in spring-operated air weapons?
Compressed spring, drives projectile
- 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
What is different about the internal ballistics in pre-charged pneumatic air weapons?
Fixed amount of compressed air
- 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
How do you test muzzle energy?
Muzzle energy = EK, maximum charge
- 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.
What can a poorly considered load lead to?
Drilled out
- 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.
What safety method is 2nd best for preventing accidental triggers?
Closer
- 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
What happens in barrel time?
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.
