It is a review paper on different kinds of Operating mechanisms in automatic rifles.Working Mechanism of mostly used automatic rifles is explained.

1. TERM PAPER
On
Rifle Mechanisms
Submitted to
Amity School of Engineering and Technology
Amity University
Uttar Pradesh
Guided By: Submitted By:
Mr. Prashant Gill Pulkit Goyal
Ms. Megha Sharma A2305414157

2. Declaration by the student
I, Pulkit Goyal, student of B.Tech (MAE) hereby declare that the
project titled “Rifle Mechanism” which is submitted by me to
Department of Mechanical and Automation Engineering, Amity
School of Engineering and Technology, Amity University Uttar
Pradesh, Noida, in partial fulfillment of requirement for the award of
the degree of Bachelor of Technology in Mechanical and Automation
Engineering, has not been previously formed the basis for the award
of any degree, diploma or other similar title or recognition. The
Author attests that permission has been obtained for the use of any
copy righted material appearing in the Dissertation / Project report
other than brief excerpts requiring only proper acknowledgement in
scholarly writing and all such use is acknowledged.
Signature
Noida
Date Name and Signature of Student

3. Certificate
This is to certify that Mr. Pulkit Goyal, student of B.Tech. in
Mechanical and Automation Engineering has carried out the work
presented in the project of the Term paper entitle "Rifle Mechanism "
as a part of First year programme of Bachelor of Technology in
Mechanical and Automation Engineering from (Amity school of
Engineering and Technology), Amity University, Noida, Uttar
Pradesh under my supervision.
Name & signature of the faculty Guide
Amity Institute of Engineering and Technology
AUUP

4. Contents:
1. Abstract
2. Introduction
3. Gas Operation
i. Cycle of Operation
ii. Long Stroke piston
4. Locking and Mechanical Safety
i. Rotating Bolt
5. Conclusion
6. National and International Work
7. References

5. Abstract
As the topic of my term paper is Rifle Mechanism, I have studied
operating mechanisms of automatic rifles. Gas operation and Rotating
Bolt is the mostly used Operating system in automatic rifles. Gas
operation is one the three basic systems of automatic operation. This
method is used to provide the automatic cycle of operation, mean
ideally that only energy that would otherwise be wasted is used. But
in practice it is not completely true because, for a given round, some
energy may lose in gas operated gun and this could produce a slightly
reduced muzzle velocity. As the pressure inside the gun is so high that
the presence of a safety mechanism, for the safety of the firer and the
gun becomes essential. This requirement is fulfilled by introducing
the Rotating Bolt mechanism in the rifle. I have discussed this
operating system of automatic rifles by taking the example of the
most used automatic rifle in the human history, AK47.

6. Introduction
Through this term paper, I want to discuss about the full operation of
the mechanisms on which the automatic rifles work. Automatic rifles
are the type of rifles, in which the rounds are chambered by
themselves only without the hectic of reloading the gun again and
again. These rifles/guns have a very high rate of fire average rate of
fire of automatic rifles is 600-700 rpm (rounds per minute).
An automatic rifle is a type of rifle which is fed by magazine. It uses
its recoil or some amount of pressure created by the gas propelling the
projectile to eject the spent cartridge case, cock the rifle, load a new
round and fire again continuously until the trigger is pressed or the
magazine gets empty.
The Browning Automatic Rifle (BAR) was one of the first automatic
rifles to be used by the army of any nation. It was introduced in
combat in the World War 1. Since then several rifles were designed,
made and used by many nations. But the Soviet Russian AK47 won
the race of automatic rifles when it came into play in early 1950s. Due
to its fire power, ease of use, low maintenance, low production cost
and reliability made AK47 the most favored rifle.
It works on the principle of Gas Operation and its mechanical safety
is ensured by Rotating Bolt system.

7. Gas Operation
In Gas operated rifles, the barrel of the gun acts as an expansion
chamber and as the projectile passes the pressure along the barrel
varies. Pressure of the gas varying from the chamber to the pressure at
the muzzle, can be drawn off. In the form muzzle blast some energy
contained within the propellant is ejected at the muzzle. A useful
source of power is provided by this energy to operate a fully
automatic system and rifles using this are known as “Gas Operated”.
To operate a piston, the gas that is tapped off in the barrel is contained
between the piston and cylinder and even if the cylinder is vented the
gas will not escape until the bullet has left the muzzle.
The factors involved in tapping the gas off near the muzzle are:
 Minimum effect on the ballistics.
 Low pressure gas requires a large volume to perform the
required task. Muzzle gases contain a lot of solid carbon which
has been re-sublimated.
The factors involved in tapping the gas off near the breach are:
 High pressure gas – therefore parts involved must be robust.
 Short delay before gas is available: this is an advantage where a
high rate of fire is required.
 Hot gases will erode the gas hole in the barrel.
These factors lead to a compromise in which the gas is tapped
off some 177-305mm form the muzzle in a rifle. This gives
required power without either unacceptable erosion or carboning
of components.

8. Cycle of operation:
 The cycle starts with the chambering of the round and locking of
the bolt to the barrel extension or to the body of the gun. When
the cartridge is fired, the projectile moves forward and those
parts of the gun designed to recoil will move back.
 As the bullet passes the port the gas will enter the cylinder and
the pressure will build up.
 After unlocking of the bolt occurs the residual barrel pressure
continues to exert force on the piston and also provide s some
blowback on the bullet case. After the unlocking, bolt is
accelerated by the piston, and the bolt and the piston move
backward together.
 The bolt extracts the fired cartridge case which is ejected from
the rifle and the return spring restores energy throughout the
rearward movement of the bolt.
 When the breech block ceases its backward travel, its kinetic
energy gets transfer to potential energy stored in the spring and
drives the bolt forward to complete the functions of feeding,
chambering and locking.

9. Long stroke piston:
 It is a type of Gas Operation. The long stroke piston is
attached permanently to the breech block and controls its
velocity and position throughout the cycle.
 The primary plus point of the long-stroke piston beyond its
simplicity of the design and its robustness is that the mass of
the rod of the piston adds to the momentum of the bolt carrier
which enables the extraction, ejection, chambering, and
locking more positively.
 Chances of the malfunctions are reduced by the longer
cleanliness of the weapon, and it is because the gas is not
directed back into the chamber.
 Due to the change in center of mass during the action cycle,
energetic and abrupt stops at the beginning and travelling of
the end of bolt carrier, it becomes difficult to aim properly.
This is a setback of long stroke pistons system.

10. Locking and Mechanical Safety:
Mechanical safety is defined as that feature of design which ensures:
 Until a round is fully supported in the chamber it cannot be
fired.
 Until the chamber pressure is at a safe level the support cannot
be removed.
Rotating Bolt
The principle of Rotating Bolt can be adapted to gas operated rifles in
which the gas piston carries a post. This rides in the hollow interior of
the cylindrical bolt and causes the bolt to rotate by bearing on a
curved cam path.
The piston post fits into the hollow interior of the bolt and bears
against the curved cam path cut into the bolt. Piston tries to rotate the
bolt as it goes forward but the shape of the body stamping prevents
this until the bolt head with the locking lugs has entered the barrel
extension. The bolt can now be rotated and the lugs move round in a
cut away section of the barrel extension and lock the bolt to the barrel.
The locking lugs give all the advantages of forward locking as they
are at the front of the bolt. Mechanical safety is obtained, before
firing; because the striker is carried on the piston post which itself is
unable to move forward along the cam groove until the bolt is rotated.
The rotation cannot occur until the locking is completed, so there is
no possibility of premature cap strike.
After firing there are two effects. Firstly, the position of the gas port
ensures that the bullet is near to the muzzle before any gas passes
through the piston wall. Secondly, the cam path has a preliminary
section parallel to the bolt axis that ensure the initial movement of the

11. piston post produces no rotation of the bolt. As a result the unlocking
action does not start until the piston extension has moved back about
32mm. By this time the bullet is out of the barrel and the pressure of
the gas greatly reduced.
The piston extension and the bolt carrier are one machining. The bolt
rides in the carrier. The carrier takes the bolt forward and the round is
chambered. When the bolt can no further move forward, the piston
extension has a further 33mm to travel and a curved cam path on the
bolt carrier causes the bolt to rotate and lock to support the cartridge
case.
Before firing, the bolt carrier must be fully forward to operate a safety
sear, which must be depressed before the hammer can be released.
The bolt carrier will have completed the bolt locking before it has
moved far enough forwards to release the safety sear. After firing, the
mechanical safety depends upon the gas port being well down the
barrel, and that the bolt moves back 6.4mm before the cam pin on the
bolt is moved across the width of the operating cam slot and also that
there is no bolt movement. When this free travel has been taken up

12. the bolt starts to rotate and by this timr the pressure is down to
atmospheric. The principle of the rotating bolt is well established.

13. Conclusions:
In this Term paper, I have discussed about the functioning of the
operating mechanism of automatic rifles. Working of the operating
system of the mostly used automatic rifle in the world, AK47 is
studied and discussed. AK47 works on the principle of Gas Operation
due to which it has a high rate of fire i.e. 600rpm. Here are the Plus
and Minus concluding points of Gas Operation rifles such as AK47.
Advantages:
 Flexibility- It is the principle advantage of Gas Operation
over other operating mechanisms. Gas operated guns can fire
under all conditions where frictional forces can become
excessive. High power is required to operate in such
condition which is catered by the gas regulator.
 Rate of Fire- A gas operated gun can have a very high rate of
fire. By tapping off gas close to the chamber and unlocking
the breech, thus obtaining blowback assistance, the cycle can
be cut down to a minimum period of time.
Disadvantages:
 Fumes-These guns throw out fumes and are therefore
not suitable to use in an enclosed space.
 Barrel Change-There is a spigot connection between
the barrel and the gas cylinder which means that the
barrel must be pushed forward to remove it from the
rifle.
It is essential for these issues to be resolved in order to make this rifle
more reliable.

14. National and International work:
As there were some issues regarding the stability of the AK47
because of which it was difficult to aim while shooting some
modifications were made in the next variants of AK47 one of the
latest variants of AK47 is AK74. It is currently used by the
Paramilitary forces of many countries including INDIA.
The Indian defence research and development agency, DRDO
(Defence Research and Development Organization) initiated a project
named INSAS for Indian Small Arms to manufacture rifles for the
Indian forces.
Its designs, up to some extent, were based on that of AK47. Later
more modifications were made according to the needs of the forces.

15. References:
 Books:
1. D F Allsop and M A Toomey (1999), Brassey’s Land
Warfare-Small Arms: General Design.
 Websites:
1. https://en.wikipedia.org/wiki/Automatic_rifle
2. https://en.wikipedia.org/wiki/Gas-
operated_reloading#Long-stroke_piston