Automatic electric guns Electrically powered airsoft guns use a spring-loaded piston pump just like spring guns, but instead of manual operation they typically use portable rechargeable battery packs to power an internal electric motor, which transmit through a gearbox to compress the pump spring and propel the pellets via the compression of air and load the pellets in a cyclic fashion . Selective fire options among automatic, 3-round burst and semi-automatic operations are all possible, which gives these guns the popular name "automatic electric guns", or AEGs. These guns often attain muzzle velocities from and rates of fire (RoF) between 100 and 1500 rounds per minute. They are the most commonly used and widely available type of airsoft gun. The AEGs were developed in Japan and the Japanese company Tokyo Marui is credited with creating the original gearbox system. In a Tokyo Marui AEG, the motor drives a train of three gears mounted inside a gearbox, which then loads a pump piston against a spring. Once the spring is released, it pushes the piston plunger forward through the pump cylinder to propel a pellet resting within the chamber forward through the barrel and out of the muzzle. Many manufacturers have now more or less replicated this basic model, adding reinforced parts or minor improvements. The electric airsoft guns were powered primarily by nickel-metal hydride (NiMH) batteries with varying voltages and milliampere-hour ratings. The most common battery is an 8.4 V large battery pack, with a capacity between 2200 and 5000 mAh. Also available are "mini" and "stick" batteries, which generally have 900 ~ 1600 mAh capacities. Voltages for NiMH batteries range from 7.2 V to 12 V. The usual rule of thumb is that the higher the mAh rating, the longer the battery lasts; the higher the voltage, the higher the rate of fire. Recently, however, the more energy-dense lithium polymer (Li-Po) batteries are becoming more popular in the airsoft world, since they last longer, have higher mAh and voltages, and can be charged more frequently without concerns for voltage depression, while at the same time being small and lightweight. Li-Po batteries are usually rated at 7.4 V or 11.1 V, and varying between 500 mAh and 6500 mAh. External modifications, such as metal bodies and reinforced plastics that make AEGs look and feel even more realistic, have become very popular. AEG manufacturers such as Classic Army and Tokyo Marui