The AT4 may be considered a disposable, lower-cost alternative to a
Carl Gustaf 8.4 cm recoilless rifle. The AT4 took many of its design features from the Carl Gustaf, which operates on the principle of a
recoilless weapon, where the forward
inertia of the
projectile is balanced by the inertia of
propellant gases ejecting from the rear of the barrel. But unlike the Carl Gustaf, 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. Being a disposable gun also allows for lighter and cheaper construction. In a single-use disposable gun, the barrel only needs to be able to contain a single pressure spike when firing, when it can be disposed of, even if it is ruined, burnt-out and strained, unlike traditional guns which are required to survive many pressure spikes without failure and thus need to be strongly overbuilt and made of heat-proof materials. Pressures are also kept quite low compared to many traditional guns. The AT4's thin, lightweight barrel, low operating pressure, and significant recoil mitigation, means that relatively large projectiles (comparable to those found in
mortars and
artillery systems) can be utilised, which would otherwise be impossible in a man-portable weapon. In the system originally developed by FFV for the Carl Gustaf, a plastic blowout plug is placed at the centre 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 adopted a unique method developed earlier by FFV: the spring-loaded firing rod is located down the side of the outer tube, with the
firing pin at the rear of the tube. When released, the firing pin strikes a primer located in the side of the casing's rim. 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 to friendly personnel in the vicinity of the user and sometimes even 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 was 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. The AT4-CS version also reduced its muzzle velocity from the original 290 m/s to 220 m/s as part of its effort to be user-safe in a confined space, making the AT4-CS version more difficult to use as the drop is more pronounced. The effectiveness of the HEAT warhead is not dependent on speed. File:M136 AT4.jpg|M136 AT4 File:M136A1 - AT4CS-RS.jpg|M136A1 AT4CS-RS File:M136A1 AT4CS-RS firing.webm|M136A1 AT4CS-RS firing To fire, the gunner first removes the safety pin located at the rear of the tube, which unblocks the firing rod. They then take a firing position, ensuring that no one is present in the back blast area. If firing from the
prone position, they must also place their legs well to the side to avoid burning themself. Then, the gunner moves back the front and rear sight covers, allowing the sights to pop up into their firing positions. The AT4 has
iron sights that were originally developed for the cancelled
Viper, and are similar in concept and use to those on assault rifles. They then remove 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 the firing button must be pressed down at the same time to fire the AT4. The red firing button has a similar resistance to the trigger pull of a rifle, so the gunner does not have to jab at the firing button, which could throw their aim off. rifle and an AT4. After firing, the AT4 launcher is discarded. Gunners are often trained to break off the sights after use, in order to prevent other soldiers from mistaking used launchers for unused ones. Unlike the heavier Carl Gustaf, the AT4 outer tube is built to take the stress of just one firing; it is not reusable and cannot be reloaded. The AT4 can mount an optical
night sight on a removable fixture. In US 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 that are identical in operation but fire reloadable
9×19mm 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 favoured 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). ==Specifications==