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:
System of levers known as
, 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.
"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.
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.
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.
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
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.
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.
Any mechanical part of a
mechanism that has a sliding contact surface.
Fire Selection Lever
Some firearms have multi-purpose control. Can select:
single fire or semi-auto fire
Controls are safe via sear engagement. Therefore are better than trigger safety but not as reliable as a firing pin safety.
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
therefore carries lower evidential weight.
2) Ignition Time
propellant type and structure
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
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
in the cartridge
until the projectile is unseated from the cartridge neck and begins to move. Graph: chamber pressure vs. time
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
phase for the projectile inside the firearm.
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)
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.
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.
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