Internal ballistics

Question 1

Lock time

Answer 1

The lock time is the time between the activation of the firing mechanism and igniton of the primer charge

Question 2

The trigger mechanism

Answer 2

• Consists of a system of levels, known as sears, which hold the firing pin or hammer under spring pressure until the trigger is operated
  1. When the firearm is ‘cocked’, the firing pin spring is compressed and then mechanically held by a sear
  2. The energy stored within the spring is then released when the trigger is pulled and is used to activate the primer

Question 3

Sear

Answer 3

• Any mechanical part of a trigger mechanism that has a sliding contact surface
• Modern triggers have multiple sears

Question 4

Firing pin safety

Answer 4

• Mechanical system that blocks the action of the firing pin
• 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 build into the bolt or slide so very hard to tamper with

Question 5

Magazine interlock

Answer 5

• Internal mechanism that engages a mechanical safety when the firearm’s magazine is removed
• Designed to prevent accidental discharge where the magazine has been removed but a round is still chambered

Question 6

Safety failure test

Answer 6

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 occured
6. Repeat 10 times noting the result of each test

Question 7

Trigger test

Answer 7

• Check the firearm is not loaded first
  1. Make sure the weapon is secured in a solid and safe position
  2. Make sure the action is cocked
  3. Use a force meter and attach it to the trigger and use it until the trigger is activated
  4. This should be repeated 10 times, noting all the results
• A drop in the force required to activate the trigger may supprt an accidental discharge or that it has been tampered with
• Tuned or custom triggers will have significantly lower operating forces - these might be serviceable but unsuited to its general application and therefore dangerous

Question 8

Ignition time

Answer 8

The time from activation of the primer charge to initial movement of the projectile
This is dependant on:
1. Primer type
2. Propellant design
3. Chamber dimensions
4. The ‘tightness’ of the bullet in the cartridge neck

Question 9

Sequence of events for the igniton time

Answer 9

1. The firing pin contacts the primer, deforming the primer cap and crushing the primer mixture against the internal anvil
2. Hot gases then pass through the flash hole and ignite the main propellant charge in the cartridge case
3. When the propellant is ignited by the primer ‘flash’ a large amount of gas is produced and the pressure in the cartridge rises quickly until it is sufficient to unseat the projectile from the cartridge neck as it begins to move

Question 10

Barrel time

Answer 10

• The time from first movement of the projectile to when the projectile exits the muzzle
• During this time, an increase in the chamber and barrel pressure continues to accelerate the projectile, this is maintained by the propellant continuing to burn until the projectile leaves the muzzle
• Barrel time is dependant 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

Question 11

How do spring-operated air weapons work?

Answer 11

• The trigger releases a compressed spring instead of activating the primer
• The spring pushes a piston which compresses air into the barrel, this drives the projectile down the barrel

Question 12

How does a pre-charged pneumatic air weapons work?

Answer 12

• Charged with compressed air from an outside source
• A valve system is activated by the trigger and allows a fixed amount of compressed air out of the storage cylinder into the barrel driving the projectile

Question 13

What can poorly considered reloading of centre-fire cartridges (hand loaders) lead to?

Answer 13

Poorly considered load can lead to:
1. The chamber pressure exceeding the design/proof limits
2. Low ignition pressure