Topic 7 - Internal ballistics Flashcards
(31 cards)
1
Q
internal ballistics
A
- When a modern firearm is discharged, a complicated sequence of
events is enacted. - This chain of events can result in chamber pressures in excess of
3500 atm, and the projectile being propelled from the barrel at
velocities in excess of 1000 m.s-1. - This all happens in less than about 0.02 seconds (20 milliseconds).
- Ballistic efficiency can be measured as a ratio of peak chamber
pressure to projectile velocity. - The internal firing process is broken down into 3 clear stages – lock
time, ignition time and barrel time.
2
Q
whats lock time?
A
- The ‘lock time’ is the time between the activation of the firing
mechanism and ignition of the primer charge. - This is mainly dependant on the design of the trigger mechanism,
which links the trigger to the firing pin assembly
3
Q
the trigger mechanism
A
The trigger mechanism is usually a wholly
mechanical system.
- 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. - There has been some experimentation with
solenoid triggers in the past, based on
electromagnets, but the mechanical systems will
prevail for now. - The trigger mechanism is normally the alleged
culprit in “accidental” shootings.
4
Q
sequence of events
A
- 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.
5
Q
whats a safety catch system?
A
- Various safety systems are employed by different manufacturers to try
and ensure that the firearm does not discharge accidentally. - Most modern firearms are required to have a ‘drop safety’ mechanism,
preventing the weapon discharging if subjected to rough handling or if
dropped. - Safety system failure is another of the most common defences offered
in cases involving shootings, so, checking whether the safety
mechanism is operational is an important part of the investigation of the
weapon when an accidental or criminal shooting. - However, in reality, the most common failure is the operator
6
Q
whats a trigger safety?
A
- 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 independent of the trigger. - Normally activated by a small button or lever located behind the the
trigger.
7
Q
what is a sear safety?
A
- This is again a mechanical safety mechanism, which this time, blocks
the action of one or more of the trigger sears. - 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 damaged by amateurs, when tampering
with the trigger mechanism. - This category includes the grip safety and fire selection levers
8
Q
what is a sear?
A
- A sear is any mechanical part of a trigger mechanism that has a
sliding contact surface. - Most modern triggers have multiple sears.
- Adjustment of these is a specialist task as getting it wrong will result
in trigger failure and an unplanned discharge.
9
Q
what is a grip safety?
A
- Very rare in the UK, since they’re usually fitted to SLPs or SMGs
- So more associated with Section 5 firearms.
- Grip safeties are integral to the weapon grip and must be compressed
before the weapon will fire. - So could this include ‘futuristic’ palm recognition safety mechanisms?
- By their nature they are always sear engagement safeties.
10
Q
what is a fire selection lever?
A
- 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
- These controls always safe the firearm via sear engagement.
11
Q
what is a firing pin safety?
A
- 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.
12
Q
what is a magazine interlock?
A
- A magazine interlock is an internal mechanism that engages a
mechanical safety when the firearm’s magazine is removed. - It is 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.
- Really should not be necessary if correct firearm discipline is maintained.
13
Q
what is an accidental discharge?
A
- There are several areas to examine if an accidental discharge is
claimed. - Some are simple mechanical tests which are well within the ability of a
competent forensic examiner.
However…
* Before any examination of a firearm is attempted you must have the
unit certified safe and unloaded by a competent person - normally a
Police FAO.
- Even then, always treat any firearm as if it is loaded.
14
Q
what is a safety failure test?
A
- By far the most commonly encountered excuse.
Test method:
1. Ensure 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.
15
Q
what is a trigger test?
A
- Always check that the firearm is not loaded first.
- Dismantling a trigger mechanism is a skilled task best left to a qualified
gunsmith…so don’t try it! - However, you can undergo a trigger pressure test to understand if the
mechanism is operating as per manufacturer pre-sets. - 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.
16
Q
what are trigger test results?
A
- 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. - Manufacturers can supply factory trigger pull values.
NOTE: tuned or custom triggers will have significantly lower operating forces. - A trigger of this type may be perfectly serviceable but unsuited to its general
application and is therefore dangerous.
17
Q
what are the typical trigger pull pressures?
A
- 0.22” rim-fire vermin rifle: 15 – 25 N
- Assault rifle (military): 20 – 35 N
- Single action revolver: 20 – 30 N
- Double action revolver: 40 – 55 N
- Semi-auto pistol: 20 – 40 N
- Shotgun: 20 – 30 N
- Professional target rifle: 2 – 15 N
- As a general rule a field or hunting firearm should have a trigger set to double the pressure of a target firearm.
- A deliberately low trigger pressure is very dangerous and has led to manslaughter charges in the past.
18
Q
case study for accidental discharge
A
- In 1992, in Powys, Wales, a 35 year old male went to trial for shooting a friend
in the head with a shotgun which was fitted with a sear safety. - The defence claimed the gun discharged accidentally whilst placing the firearm
in the rear of the victim’s car. - The firearm was tested a total of 100 times with gradually increasing impact
force and no trigger failure occurred during any of the tests. - However, when dismantled it was found that there was a broken spring on the
safety engagement sear. - Although the firearm worked perfectly during all the tests the broken spring was
enough to introduce a doubt as to the reliability of the trigger/safety mechanism. - The case was dismissed.
19
Q
what is ignition time?
A
- The ignition time is the time from activation of the primer
charge to initial movement of the projectile. - This is dependant upon:
1. Primer type
2. Propellant design
3. Chamber dimensions
4. The ‘tightness’ of the bullet in the cartridge neck.
20
Q
what are the sequence of events for the ignition time?
A
- 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
21
Q
what are the sequence of events for the ignition time? part 2
A
- Remember that firearm propellants do not typically detonate
(explode); they deflagrate (burn) at a controlled rate. - 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.
- 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.
22
Q
what is barrel time?
A
- The ‘barrel time’ is the time from first movement of the projectile to
when the projectile exits the muzzle. - During this time, an 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. - Barrel time is dependent on:
1. Projectile shape, size (caliber), and mass
2. Internal barrel friction
3. Rifling type and rate of twist
4. Chamber pressure and propellant burn rate - This is the acceleration phase for the projectile inside the firearm.
23
Q
ideal gas law
A
PV = nRT
24
Q
Boyle’s law
A
P1V1 = P2V2
25
which equation dictates the force on a projectile?
F = P x A
26
what are projectile forces?
* During the barrel time, the projectile experiences massive forces.
* If a 5.56 mm, 3.6 gram bullet is propelled from rest to 1200 m.s-1 along a
0.5 m long barrel, calculate the acceleration…
* It is also subject to rotational forces which spin it from zero to 240,000
revolutions per minute in the same distance and time.
27
what are the advanatages and drawbacks or airweapons using spring operated mechanisms?
* Despite not having a cartridge-based system. all the stages of internal ballistics
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, which drives the projectile down the barrel.
So we have:
* Lock time
* No ignition time, but compression time
* Barrel time
* The biggest danger (aside from shooting yourself or others) is injury from the
mechanism itself.
28
advanytages and drawbacks or pre charged pneumatic air weapons?
Pre-charged pneumatic (PCP) air weapons are charged with compressed air from
an outside source such as:
o Diver’s Bottle
o Foot pump
o CO2 cartridge.
* The operating pressure of
these units can be in excess
of 220 Bar (217 atm).
* 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.
* With this type of air weapon we have:
o Lock time
o No ignition time, but valve operation time
o Then barrel time
29
air weapon energy value laws
* Below 12 ft.lb (or 16.32 J) of muzzle energy, air weapons are
unlicensed in terms of UK firearms law.
* Above these values, air weapons must be treated as either a Section 1
firearm (air rifles) or section 5 (air pistols), needing the necessary
certificates to own.
* So, energy output is crucial when it comes to
determining the legal status of the system.
* So how do we test the muzzle energy?
30
what is the home office test?
* 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 (or similar).
31
what are hand loaders?
* Reloading of centre-fire cartridges (or homemade
ammunition) is very common amongst firearm
enthusiasts and requires skilled operation.
A poorly considered load can lead to:
1. The chamber pressure exceeding the
design/proof limits.
* In this case, the firearm can explode.
2. 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. Attempts to clear by
firing a second round will result in the same end as
case 1 above.
