The M30 106.7 mm (4.2 inch, or "Four-deuce") heavy mortar is an American rifled, muzzle-loading, high-angle-of-fire weapon used for long-range indirect fire support to infantry units. Design The M30 system weighs including the complete mortar with a welded steel rotator, M24A1 base plate and M53 sight. A point of interest in the design of this mortar is the rifled barrel. A rifled barrel requires the round to be a very tight fit to the bore in order for the rifling to engage the round and impart rotation to it. But, in a muzzle-loading mortar, the round has to be loose enough in the bore to drop in from the front. In order to have it both ways, these rounds have an expandable ring at the base, which expands into the rifling under the pressure of the firing charge that propels the round. Additionally, imparting a spin to a round causes it to drift away from the direction of fire during flight and the longer the flight (greater range to target), the farther the drift, so the computation for setting the direction for firing at a specific target has to account for this drift. American rounds are designed to be drop-safe and bore-safe. As such, the fuzes in the rounds for this rifled mortar only armed once the round had spun a certain number of times i.e. the round was not armed until it had exited the barrel spinning, and had consequently traveled a safe distance from the gun emplacement. Types of rounds HE M329A1—max range , weight HE M329A2—max range , weight WP M328A1—max range ILLUM M325A2—max range , 90-second burn time ILLUM M335A2—max range , 70-second burn time ILLUM is illumination, a parachute flare round with fixed timed detonation. Deployment height above ground is determined by gun elevation angle and propelling charge. HE (high-explosive) and WP (white phosphorus) rounds could be fitted with various fuses before firing, including a proximity fuse set for detonation at about 30 feet above ground to maximize the affected target area and to spray shrapnel down into foxholes. There was also a sub-caliber training device that utilized blank 20-gauge shotgun shells to propel an inert training round a few hundred meters. This training was for the gunnery skill of laying (in a sense, aiming) the guns. This device had originally been developed during WWII for the M2 mortar. History The M30 entered service with