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The 25 x 40mm grenade is a type of explosive ordnance used in grenade launchers, designed for longer range, flatter trajectories, easier feeding into weapons, and better aerodynamics.

Examples of weapons designed to launch 25 mm grenades are the XM25, the Barrett XM109 payload rifle, and the XM307 ACSW. The original use for this round was for the OCSW, now known as the XM307 ACSW. Later, a low velocity variant of the OCSW round was chosen for the XM25, which is derived from the XM29 OICW. There are two different versions of the 25mm grenade, the 25 x 40mm low-velocity round developed for the XM25, and the 25 x 59mm high-velocity round developed for the XM109 and XM307 ACSW.

25 mm grenades are armed after launch, after traveling a safe distance. Many types explode on impact, but others are designed to explode in mid-air after traveling a certain distance—used to clear entrenched or barricaded positions.

The weapon is essentially the same as the XM25, except that it fires caseless 25mm grenades, which are roughly half the weight and have somewhat reduced recoil.

The XM25 CDTE, or XM25 Counter Defilade Target Engagement (CDTE) System, fires 25 mm grenades that are set to explode in mid-air at or near the target. A laser rangefinder in the weapon is used to determine the distance to the target. The user can manually adjust the detonating distance by up to 10 feet (3.0 m) shorter or longer; the XM25 automatically transmits the detonating distance to the grenade in the firing chamber. The grenade tracks the distance it has traveled by the number of spiral rotations after it is fired, using a magnet to measure the magnet field of the earth, and then detonates at the proper distance to produce an air burst effect. These features make the XM25 more effective than traditional grenade launchers at the task of hitting targets that are behind cover or dug into the ground (i.e. in defilade.) In general, all 25mm x 40mm grenade possess these capabilities, although not all systems are designed to exploit their air bursting capabilities.

The rounds are designed with the same type of propellant as the 3GL, and similiar to that in the LSAT. As a result, the rounds can be caseless, which drastically reduces their weight and doesn't require the ejection of shells. The rounds are also detonated electrically, eliminating the need for a complex firing system, or a reciprocating bolt.