机械工程英语作业——翻译.doc

Russia SVD Sniper Rifle
In 1963,the former Soviet union army chose sniper rifle designed by Dragonov ,known as CB Д sniper rifles,whose English name is SVD (Snayperskaya Vinyovka Dragunov), according to further improvement,finally to be equiped for the army in 1967, instead of Mosin-Nagant sniper rifle.

Summary
In 1958,the Soviet Union put forward the idea of designing a semi-automatic sniper rifle, required to improve the firing accuracy, functional reliability in a harsh environment, as well as simple and compact structure. After 5 years,former Soviet union army chose the semi-automatic sniper rifle designed by Dragonov. In 1967,it started to appear in army by further improvement.The new modified SVD adopts the new glass fiber composites gunstocks and apron, and a new magazine,in front of the entrance to which has threaded holes to Install the bipod.
In the Soviet army, each class is equipped with a SVD. And soldiers armed SVD receive special training for the weapon.Due to its simple design is for the purpose of extending the range of classes, SVD is a rugged rifle.Iron Sights for aiming is further proved out the truth ,under this circumstance that bayonet holder and sighting telescope are damaged.SVD reliability is still recognized (Russian light weapons are generally considered very durable), which makes the SVD has been long and widely used.
In addition to the former Soviet Union / Russia, the armies of Egypt, Yugoslavia, Romania and other countries have adopted and production SVD.The postiche SVD in China is type 79 sniper rifle that has finalized the design and Improved type 85.

Structural features　
SVD firing mechanism can actually be seen as a larger version of AK47 assault rifles, but more simpler. Due to the 7.62 × 54mm flange bomb launched by SVD,is more powerful than the 7.62 × 39mm M43 missile from AK47, so that the bolt head needs to be redesigned and the SVD sniper rifle needs to be strengthened to withstand high pressure.However, because of only a single shot, so firing and firing mechanism is relatively simple, the main parts are the hammer, single lever as well as insurance resistance iron by the control of machine frame , separate hammer spring and the trigger spring. This gun can use the old Mosin - Nagant M1891/30 ammunition, but it typically uses a more accurate 7N1 bombs made for SVD specially.
　　 The air piston’s difference between AK47 and SVD is that, the former is a whole with the bolt box and the latter adopt short-stroke piston design,what’s more, the air piston separately located in piston cylinder moves backwards under the gunpowder gas pressure, after it impact machine frame to make it sit ,so you can lower the offset of the center of gravity caused by the movement of the piston and piston rod , thereby improving shooting accuracy.Unlocking principle when machine frame recoil is the same with AK,as well as all shelling, complex operation and loading after unlocking.Handguard is not designed in direct contact with the barrel, but fixed on the casing. There is petal-shaped flame trap in front of the barrel, 70mm length,5 slots, three of which located in the upper, and 2 at the bottom.In this way, the expellant gas from the upper flame trap is more than from the bottom,whose actual effect is the muzzle down to reduce muzzle jump ,to some extent .Below quasi-constellation ,there is a bayonet base that can be mounted bayonet ,which is not the same with the vast majority sniper rifle.

SVD gunstocks design is generally hollowing out the rear of the wooden gunstock grip and the majority of gunstock, not only reducing weight, but also naturally forming straight grip. At the same time, gunstocks can better control the muzzle jump,with it’s centroid more close to the barrel axis. On the gunstock of length non-adjustable, there is a detachable cheek. Afterwards,the production of SVD use glass fiber composite material stock. SVD trigger retainer is relatively larger ,and soldiers can also shoot with cotton or leather gloves.　
　The standard gun sights of the SVD is 4 x 24 mm PSO- 1 sights,at the 375 mm lens, field of 6°.Although the magnification of PSO - 1 sights is only four times, the range adjustment nut can make ballistic correction to 1000 m. Considering of three distance divisions on the reticle of the sights and 100 m of each division, the maximum range may be up to 1300m. There are light source and the battery on the reticle to illuminate it at night,in addition,a kind of infra-red filter with gun sight is applied to filter the external infrared light when shooting at night, but gun sights itself does not have the ability of night vision. As a result of SVD casing is AK type, thus, the installation of the sights only installs in the left side of the casing.

1-object slide；2-objective；3-color filter；4-the hand shank；
5-Adjust the nut；6-scale；7-lens body；8-eyepiece；
9-Rubber protective eyewear；10- battery coverall；11–lens cap

Ammunition
7.62 x 54 mm R bullets in fact, have begun to be used from the Russian empire,and they are now usually only used as machine guns.However, the SVD apply the same specification but specially developed sniper bullets, with higher precision,bigger power and strong lethality even in the 1000 m distance ,as well as special armor-piercing incendiary bombs tracer. Certainly,SVD can also launch regular 7.62 x 54 mm R machine bullets. In fact, SVD early design of bound distance is 320 mm, later reducing to 240 mm. The change causes the poor accuracy while launching regular play, and decreased muzzle velocity , from 830 m/S to 810 m/S.Doing this mainly matches up with maximum rotation speed of newly developed special sniper bullets and armor-piercing incendiary bombs tracer,to improve ballistic performance of these special ammunition fired,so muzzle velocity is still 830m / s when the SVD launch 7N1 bomb.