Topic 7 - Internal ballistics

Question 1

internal ballistics

Answer 1

• 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.

Question 2

whats lock time?

Answer 2

• 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

Question 3

the trigger mechanism

Answer 3

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.

Question 4

sequence of events

Answer 4

• 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 5

whats a safety catch system?

Answer 5

• 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

Question 6

whats a trigger safety?

Answer 6

• 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.

Question 7

what is a sear safety?

Answer 7

• 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

Question 8

what is a sear?

Answer 8

• 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.

Question 9

what is a grip safety?

Answer 9

• 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.

Question 10

what is a fire selection lever?

Answer 10

• 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.

Question 11

what is a firing pin safety?

Answer 11

• 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 12

what is a magazine interlock?

Answer 12

• 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.

Question 13

what is an accidental discharge?

Answer 13

• 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.

Question 14

what is a safety failure test?

Answer 14

• 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.

Question 15

what is a trigger test?

Answer 15

• 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.

Question 16

what are trigger test results?

Answer 16

• 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.

Question 17

what are the typical trigger pull pressures?

Answer 17

• 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.

Question 18

case study for accidental discharge

Answer 18

• 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.

Question 19

what is ignition time?

Answer 19

• 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.

Question 20

what are the sequence of events for the ignition time?

Answer 20

• 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 21

what are the sequence of events for the ignition time? part 2

Answer 21

• 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.

Question 22

what is barrel time?

Answer 22

• 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.

Question 23

ideal gas law

Answer 23

PV = nRT

Question 24

Boyle’s law

Answer 24

P1V1 = P2V2

Question 25

which equation dictates the force on a projectile?

Answer 25

F = P x A

Question 26

what are projectile forces?

Answer 26

• 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.

Question 27

what are the advanatages and drawbacks or airweapons using spring operated mechanisms?

Answer 27

• 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.

Question 28

advanytages and drawbacks or pre charged pneumatic air weapons?

Answer 28

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

Question 29

air weapon energy value laws

Answer 29

• 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?

Question 30

what is the home office test?

Answer 30

• 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).

Question 31

what are hand loaders?

Answer 31

• 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.