FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III
Subject FORENSIC SCIENCEPaper No and Title Paper 6: Forensic Ballistics
Module No and Title Module No. 8: Internal ballistics III
Module Tag FSC_P6_M8
TABLE OF CONTENTS
1. Learning outcomes 2. Introduction
3. Recoil of the weapon 3.1. Elements of recoil
3.2. Measurement of Recoil of firearm 3.3. Recoilless Guns
3.4. Recoil & Crime Investigation 4. Barrel failures
4.1. Obstruction
4.2. Rusting or corrosion 4.3. Erosion
5. Hang fire 6. Summary
FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III 1. Learning Outcomes
After studying this module, you shall be able to
· Learn the factors which affect the internal ballistics
· Know how the recoil of the weapon is reduced
· Identify the causes of barrel failure
· Evaluate the concept of hang fire
· Precautions to be taken in case of hang fire
2. Introduction- Internal ballistics
(a) Internal ballistics is the study, which deal with the motion of a projectile/projectiles in the bore of the weapon. Some of the factors namely lock time, ignition time, barrel time, and combustion of propellants, Ignition, density of loading, and problems of heat, velocity of projectile, pressure in the barrel along with bullet jump and durations have already been discussed in part I and II.
(b) Additional factors, which concern internal ballistics, are described below:
· Recoil of the weapon.
· Failure of barrel due to
o Obstruction in the bore o Burnt barrel
o Rusting, corrosion and erosion
· Hang Fire
3. Recoil of the Weapon
The phenomenon of backward push of the weapon is called recoil and kick is the effect of the recoil. When a cartridge gets fired, the propellant burns and produces a large volume of gas, which exerts pressure in all directions within the combustion chamber. The pressure on the bore of bullet propels it out of bore.The pressure on the side expands the case to the limit that it is a tight fit against the chamber wall and the pressure on the base of the cartridge case, makes the whole weapon in the backward direction and the shooter gets a kick. The Phenomenon of weapon getting a backward push (opposite to the direction of the projectile) is recoil and the shooter will get a kick on fire.The entire episode is governed by the Newton’s third law of motion, which states that:
FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III
“To every action, there is an equal and opposite reaction”
In terms of mathematics it is expressed that momentum of bullet and momentum of gun are equal and opposite, i.e. MV= M1V1
Where,
M Represents the mass of the gun V Represents the velocity of recoil M1Represents the mass of the projectile
V1Represents the velocity of the projectiles
It indicates that weapon and the bullet will have equal velocities, if both of them have equal weight (or mass). In actual practice, the weapon has weight 300 times the weight of bullet hence the weapon recoils with a velocity, which is very small ( 1
300 of the velocity of bullet) in comparison to the velocity of bullet. It is on this account that a gun is held tightly against the shoulder so that a part of body weight is added to the weapon with the result that the recoilgetconsiderably reduced further. If, however, a shoulder firearm is held loosely in a contact, the recoil can bruise the shoulder.
Fig 1: Showing the Bruised Arm
It has already been discussed that the recoil velocity V can be calculated when mass M of the weapon is known along with velocity V1of the bullet and its mass M1 using
1 1
or 1 1
MV M V V M V
M
=
=
FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III
In other words the recoil velocity of the gun equals the bullet velocity, times the bullet mass divided by the mass of the gun.
Example: A gun weighing 2 pounds firing a 158 g bullet at velocity 860 feet/second will have recoil velocity of gun equal to 860X158
700X1
2 [1pound = 7000gr] = 9.7 ft/sec, which is the recoil velocity of gun.
In these calculations, one thing, which has not been taken into consideration and neglected,is the mass of propellants. In cartridges loaded with nitro celluloses propellant the mass is very small and adds little to the overall figure. However, the mass is considerably greater and the effect quite pronounced when cartridge is loaded with black powder. This illustrates why a cartridge loaded with black powder propellant will give much greater recoil energy than one with the same bullet mass and bullet velocity but loaded with nitrocellulose propellant.
In simple terms, the recoil velocity is the factor, which makes a gun unpleasant to shoot. The higher the recoil velocity, the greater the discomfort.The figure of 15ft/sec was given long back (in 1929) as the highest recommended recoil velocity of a rifle. Above this figure, “gun head ache” would be experienced.
3.1Elements of recoil
There are three main causes due to which recoil of the weapon takes place and they are:
(i) Muzzle velocity of the projectiles consisting of forward motion of ejecta consisting of bullet, wads and powder residue particles. The more the velocity of ejecta, the greater the recoil of the weapon.
(ii) The motion of outgoing gases
(iii) The muzzle blast, which pushes the firearm backward in the same manner as the blast, propels a jet aeroplane or a rocket.
About 50% of the contribution towards the recoil of the weapon of the projectile is made by the motion of projectile, about 10% by the motion of gases and about 40% by the muzzle blast.
3.2Measurement of Recoil of firearm
There are several methods used by the manufacturers of firearms for measuring the recoil of a firearm and some of them are briefly described below:
FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III
(a) Use of high speed camera
Fig 2: High Speed Camera
A high-speed camera is used to note the recoil velocity or backward movement velocity of the firearm suspended from fine wires, when fired.
(b) Ballistics Pendulum method
Fig 3: Ballistics Pendulum
The firearm is fixed to a ballistics pendulum and fired. The movement of the pendulum is measured and recoil velocity ascertained from the momentum.
(c) Dynamometer method
Fig 4: Dynamometer
FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III
Compression of a spring or depth of indentation in lead is
measured by a dynamometer and compared with the standard or known compressions or indentations.
A gun of about 50 pounds weight is suspended from wires of about 10 feet length and fired. The arc made by the gun after firing is noted and used for calculating the recoil.
3.3 Recoil-less Guns
Such guns do not exhibit any recoil since the same has been completely eliminated by the technique described below.
v The cartridge case for such a gun is perforated with a number of holes
v The chamber size is considerably increased and made much larger than the cartridge v The chamber and the breech have a large number of holes with controlled directional
out lets
v The out coming gas-jets thereby take definite directions so that the rotation of the gun due to rotation of the projectile inside the barrel is compensated
v Proper adjustment of holes which ensure that the gun is propelled forward by the jets only to that extent which will neutralize the backward movement due to recoil v Thus the recoil phenomenon becomes absent and the gun does not show any recoil.
The recoil can be reduced. If the cut compensators are used, most of the recoil due to the blast is eliminated. Muzzle brake devices can also reduce the recoil of the firearm.
Fig 5: (a) Cut compensator (b) Muzzle break
Recoil & Crime Investigation
(i) Recoil has played a very important role in criminal investigation in cases of suicide as well as homicide
FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III
(ii) The improvised and homemade 12 bore guns (pipe guns) are frequently used by criminals using standard ammunition. Many times they cause injuries to shooters who get identified from the position of injuries and their characteristics.
(iii) Novices in the crime using either standard or non-standard 12 bore guns for the first time is likely to get injured during the process of the recoil especially when the gun is fired with a loose contact with his body. The telltale recoil injuries thus form important evidence.
(iv) At times, the pipe guns (improvised weapons) burst resulting in injuries to the shooter because of inadequate strength of the barrel.
Pipe gun
The suspect shooter gets identified, very easily. Thus it becomes clear that recoil of the weapon plays an important role in criminal investigation hence the ballistic experts need to understand recoil of weapons to assist investigators at the scene of crime and with examination of fire arms in forensic laboratories.
4. Barrel failures
4.1 Obstruction
Hatcher has reported cases of high-powered rifles being blown up by firing new weapons without making the bore of the thick grease in the bore although they were not having any manufacturing defect in them.
In a case reported from rural area of Muzaffarpur district an elephant ran amuck. A young licensee took out his shot gun and fired at the rouge with cartridges loaded with pellets. The pellets were unable to stop the elephant and as a result the elephant started chasing the shooter.
In an attempt to save himself, he ran but fell down with his gun during which a quantity of mud got stuck towards the muzzle-end of the weapon. He got up and again fired the gun, which on account of obstruction in the barrel got burst. The elephant, thereafter, crushed the shooter under his feet.
4.2 Rusting or corrosion
FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III
1. The life of a high powered riffle barrel is expressed in
terms of the number of rounds it is capable of shooting without affecting the accuracy. This number is three thousand to five thousand. The stipulated capacity of barrel is reduced by neglect or careless handling resulting at times in accidents.
2. Rusting or corrosion from lack of cleaning is the frequent cause, which shortens the life of a barrel. Damage from improper cleaning ranks next.Rusting is oxidation and, therefore a slow combustion. Corrosion is the phenomenon of disintegration of metal by electrochemical reaction or by direct chemical attack. Atmospheric corrosion is an example of chemical reaction. Gases in the atmosphere especially around industrial area affect the metal in general, and barrels. Atmospheric humidity when high will cause rusting.
4.3 Erosion
Erosion in a rifle barrel is due to the effects of thermal and mechanical stresses and the jet effects of hot powder gases which accentuate the mechanical stresses to wash metal away. Thus the bullet may yaw slightly hitting the rifle at an angle and this distorts and upsets it. Yawning affects the stability of the bullet in flight and accuracy is disturbed. The effect of erosion results in loss of grouping ability at long range.
Fig 6: Showing Yawning Effect of Bullet
A.O. 300 magnum rifle was manufactured out of chrome-molybdenum steel and use of one a gun 1000 yards bull gun with maximum powder charge. The throat diameter adjacent to the chamber neck had been eroded and the original diameter of 0.309 inches had been enlarged to 0.345 inches. The maximum portion of the throat also got enlarged. The whole throat was roughened, reached and burned black like charred wood.
5. Hang fire
A hang fire can be defined as the unusual time interval in the functioning of a propelling charge at the time of firing. The amount of delay is unpredictable but in most of the cases will fall within the range of a fraction of a second to a few minutes.A hang fire cannot be distinguished from a misfire.
FORENSIC SCIENCE PAPER No.6:FORENSIC BALLISTICS MODULE No. 8:INTERNAL BALLISTICS III
In a case reported from the rural area of one of the states of India, an enthusiastic teacher borrowed the gun of a local zaminder with the sole intention of showing respect to the National flag after its hoisting on an independent day (15th August). He loaded the weapon with a live cartridge, pointed the muzzle towards the sky and snapped the trigger. The cartridge failed to fire. He immediately made the gun horizontal presumably to extract the cartridge out of the gun for reloading another cartridge. The gun fired and a boy standing in front of it got killed. It was a case of hang fire and the teacher had no idea about a hang fire.
Precautions
v In cases of hang fire and misfire, the gun should never be opened immediately after one notices it to be a case of hang fire/misfire
v The shooter should be patient and wait for some time.
v How long should he wait is a question.The answer to the question is not definite but it must be less than a minute in any case but preferably be a few minutes
v While opening the gun, it must be ensured that nobody is in front of the muzzle of the weapon.
5. Summary
v Internal ballistics is the study, which deal with the motion of a projectile/projectiles in the bore of the weapon.
v There are three factors which affects the internal ballistics- recoil of the weapon, barrel failure and hang fire.
v The phenomenon of backward push of the weapon is called recoil and kick is the effect of the recoil.
v Barrel failure is caused due to obstruction in the bore, Rusting, corrosion or erosion.
v A hang fire is the unusual time interval in the functioning of a propelling charge at the time of firing.
