The AT4 may be considered a disposable, low-cost alternative to a Carl Gustav recoilless rifle. The AT4 took many of its design features from the Carl Gustav, which operates on the principle of a recoilless weapon, where the forward inertia of the projectile is balanced by the mass of propellant gases ejecting from the rear of the barrel. But unlike the Carl Gustav, which uses a heavier and more expensive steel tube with rifling, the disposable AT4 design greatly reduces manufacturing costs by using a reinforced smoothbore fiberglass outer tube. In a recoilless weapon, the barrel does not need to contend with the extreme pressures found in traditional guns and can thus be made very lightweight. This fact, combined with the almost complete lack of recoil, means that relatively large projectiles (comparable to those found in mortars and artillery systems) can be utilized, which would otherwise be impossible in a man-portable weapon.
In the system originally developed by FFV for the Carl Gustav, a plastic blowout plug is placed at the center rear of the shell casing containing the projectile and propellant, which itself is enclosed in the AT4 outer tube. When the gases build up to the correct pressure level, the blowout plug disintegrates allowing the proper amount of gases to be vented to the rear, balancing the propellant gases pushing the projectile forward.
The AT4 uses a unique method developed earlier by FFV and adopted for the AT4: the spring-loaded firing rod is located down the side of the outer tube, with the firing pin at the rear side of the tube. When released, the firing pin strikes a primer located in the side of the casing's rim. Additionally, as the shell casing absorbs the majority of the firing stresses, the launch tube can be designed to be very lightweight as it does not have to contend with the extreme pressures found in traditional cannons.
The disadvantage of the recoilless design is that it creates a large back blast area behind the weapon which can cause severe burns and overpressure injuries both to friendly personnel in the vicinity of the user and sometimes to the users themselves, especially in confined spaces. The back blast may also reveal the user's position to the enemy.
The problem of back blast has been recently solved with the AT4-CS (Confined Space) version, specially designed for urban warfare. This version uses a saltwater countermass in the rear of the launcher to absorb the back blast; the resulting spray captures and dramatically slows down the pressure wave, allowing troops to fire from enclosed areas. It should be noted that the AT4-CS version also reduced its muzzle velocity from the original 290 mps to 220 mps as part of its effort to be user safe in a confined space, making the AT4-CS version less effective.
To fire, the gunner first removes the safety pin located at the rear of the tube, which unblocks the firing rod. He then takes a firing position ensuring that no one is present in the back blast area. If firing from the prone position, he must also place his legs well to the side to avoid burning himself. Then the gunner moves back the front and rear sight covers, allowing the sights to pop up into their firing positions. The AT4 sights are iron sights and were originally developed for the cancelled Viper, and are similar in concept and use to those on assault rifles. He then removes the first of two safeties by moving the firing rod cocking lever (located on the left side) forward and then over the top to the right side. The gunner takes aim, while at the same time holding down the red safety lever located in front of the cocking lever, and then fires by pressing forward the red firing button with his right thumb. Both the red safety lever and firing button must be pressed down at the same time to fire the AT4. The red firing button has resistance similar to the trigger pull of an assault rifle, so the gunner does not have to jab at the firing button which could throw his aim off.
After firing, the AT4 is discarded. Unlike the heavier Carl Gustav, the AT4 outer tube is built only to take the stress of one firing; it is not reusable and cannot be reloaded like the Carl Gustav.
The AT4 can mount an optical night sight on a removable fixture. In U.S. military use, the launcher can be fitted with the AN/PAQ-4C, AN/PEQ-2, or the AN/PAS-13 night sights.
The AT4 requires little training and is quite simple to use, making it suitable for general issue. However, as the cost of each launcher makes regular live-fire training very expensive, practice versions exist which are identical in operation but fire reloadable 9mm or 20mm tracer ammunition. Both practice cartridges are unique to their respective weapons, with their trajectory matched to that of the live round. The 20mm version also has a recoilless weapon effect with the same high noise and back blast as the AT4 firing and is favored by the Swedish army because of the added realism of the back blast as compared to the "plonk" sound of the 9mm round (similar to the sound of a finger tapping on an empty can).
|AT4 version adopted by US as M136|
|AT4 launcher shown with ammunition and HEAT projectile|
|AT4 cutaway illustration showing ammunition installed|
|Cutaway of AT4 HEAT warhead - note trumpet shaped liner and focus ring at front of liner|
|AT4 front sight extended|
|AT4 rear sight extended|
|AT4 cock safety, press finger safety, and thumb firing button|
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