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Topic 5 - Internal Ballistics (Safety (Safety Catch: "Drop…
Topic 5 - Internal Ballistics
This is the mechanism by which firearms are discharged. The study of the events between trigger being pulled and the bullet leaving the firearm.
1) Lock Time
This time between the activation of the firing mechanism and ignition of the primer charge.
trigger --> lock time --> activation of primer
The Trigger Mechanism:
Mechanical
System of levers known as
sears
, which hold the firing pin or hammer under spring pressure until the trigger is operated.
Normally the alleged culprit of "accidental" shootings.
Sequence of Events:
firing pin is restrained mechanically (safety mechanism). Its held in rearward position until trigger is pulled.
When the firearm is cocked (pulling back the slide), the firing pin spring is compressed.
This stored energy is used to activate the primer.
When the trigger mechanism is operated the firing pin is released and strikes the primer.
Safety
Safety Catch:
"Drop safety" mechanism.
This prevents weapon disharching due to rough handling or if dropped.
Saftey catch failure is a v common defense in shootings.
Most common failure is the operator.
Trigger Safety:
Mechanism that physically blocks the trigger action.
The external trigger is blocked however the rest of the firing mechanism (particularly the dears) are free to operate independent of the trigger.
Sear Safety:
Mechanical system
Blocks the action of one or more trigger sears (levers).
More reliable than trigger safety but failure will allow the firing pin to move.
Includes grip safety catches and fire selection levers.
Commonly damaged by amateur tampering with the trigger mechanism.
Grip Safety:
Rare in UK
Pistols and SMGs (section 5 firearms)
These are integral to the weapon grip and must be compressed before firing.
Always sear engaged safeties
Not reliable.
Firing Pin Safety:
Mechanical system that blocks the action of the firing pin.
Most reliable and effective system as it is unaffected by the operation of the action, trigger or trigger sears.
Independent of trigger system, build into bolt of slide so v hard to tamper with.
Magazine Interlock:
Internal mechanism engages a mechanical safety when the firearm is removed.
Designed to prevent accidental discharge where the magazine has been removed but a round is still chambered.
Forensic testing of the mechanism will therefore require a magazine to be installed.
Sear
Any mechanical part of a
trigger
mechanism that has a sliding contact surface.
Fire Selection Lever
Some firearms have multi-purpose control. Can select:
safe
single fire or semi-auto fire
burst fire
automatic fire
Controls are safe via sear engagement. Therefore are better than trigger safety but not as reliable as a firing pin safety.
Accidental Discharge
Test this through simple mechanical tests:
make sure firearm is unloaded.
cock the action and dry fire repeatedly.
re-cock then impact the firearm is all places (back, top and side).
operate trigger with safety enaged
operate firearm same as the accident occured
repeat 10 times.
This is a
subjective test
therefore carries lower evidential weight.
2) Ignition Time
Depends on:
primer type
propellant type and structure
chamber dimensions
This is the time from the activation of the primer charge to initial movement of the projectile.
Sequence of Events:
Firing pin contacts primer and deforms the primer cap crushing the primer mixture against an internal anvil.
Primer mixture is a primary explosive (lead styphnate) and the
pressure causes it to detonate
.
Resultant hot gases pass through the flash hole and ignite the main charge in the cartridge case.
Primers are manufactured in a variety of "power" ratings.
Burn vs Bang
Propellants
deflagrate
i.e. they burn at a controlled rate.
Burn rate is determined by
grain size/shape, primer type and chamber dimensions
.
Burn rate is chosen carefully for each application.
When propellant is ignited by primer flash (the bang), the
pressure
in the cartridge
rises rapidly
until the projectile is unseated from the cartridge neck and begins to move. Graph: chamber pressure vs. time
Propellants:
Burn rate depends on:
Grain size: large surface area to mass ratio means more reaction area.
Grain shape: increased reaction area and focused ignition.
Packing density: case to propellant charge volume ratio.
Chemical moderation: specifically tailored to maintain chamber pressure as projectile moves down barrel.
3) Barrel Time
This is the time from the first movement of the projectile to when the projectile exit the muzzle.
It is the
acceleration
phase for the projectile inside the firearm.
Depends on:
caliber
projectile shape, size and mass
internal barrel friction
rifling type and rate of twist
chamber pressure and propellant burn rate.
v = x m/s
u = 0 m/s
s = x m/s
a = (v^2 - u^2) / (2 x s)
Air Weapons
Spring:
All stages are present. However, the trigger releases a compressed spring instead of activating a primer. The spring pushes a piston which compresses air into the barrel.
Compression time
rather than ignition time.
Pre-charged and CO2:
Weapons are charged with compressed air from an outside source e.g. pump. A valve system allows a measured amount of compressed air into the barrel, driving the projectile.
Valve operation time
rather than ignition time.
Legislation:
If air weapon is below 16 J then no licence is required, therefore power output is crucial, under limit = no offence.
Use chronograph to measure velocity using heavy medium and light pellets. Then use E = 1/2 x mv^2