These grenades are either projectiles, fired from a grenade launcher such as the M203 or GP-25, or throwable. The shells work on the system of an internal piston, filled with gas. Either a series of BB's or in some cases a rubber or soft foam head is seated in or on top of the shell. When the pressure is released the projectile(s) are shot from the launcher sent downrange. In the case of the throwable grenades, inside the grenade there is a similar piston to the one used in the shells, but is on a literal "timer" that allows the user to clear the area of effect. BB's or powder act as the projectile in the case of these grenades. Currently both types of grenades are not very common, mostly because grenade launchers are quite expensive and the throwable grenades are not very reliable. High pressure air systems High pressure air (HPA) systems are a type of pneumatic airsoft weapon that use externally supplied high pressure air instead of internal gas canisters like the majority of gas-operated airsoft guns. They work by using a separate high-pressure air tank that is connected to the airsoft gun with a hose, which is connected with a pneumatic motor inside the gun (called an "engine") at where the gearbox would be in a normal electric airsoft gun. The engine is powered by a fire control unit that can adjust to the desired rates of fire as well as the dwell that determines how much air is released with each individual shot. There are several types of HPA systems and they vary in both price and performance. Popular HPA brands and engines include PolarStar (Fusion Engine, F1, Jack), Wolverine (Hydra, Bolt, Inferno, Wraith, SMP), Valken (V12), and Tippmann (M4 Carbine) It is not definitive which style is more effective. It is more up to personal preference. Hybrid guns Hybrid airsoft guns are the newest type of airsoft guns on the market. They are basically standard AEGs or GBB guns with a "little extra reality" built in, and are usually more powerful. AEG hybrid operation: The magazine is loaded with shell cases, each containing a single plastic pellet. These shell casings can have a small red cap, the same as those found in any child's toy cap gun placed on the top of them. These hybrid guns feature an electrically powered, full blowback system and operate