M30 mortar The M30 107 mm ( 4 . 2 inch Mortar ) 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 305 kg 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 explosion of the firing charge which 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 did not arm unless the round was spun a certain number of times i . e . the round was not armed until it had exited the barrel spinning and traveled a safe distance from the gun emplacement . Types of rounds . WP is white phosphorus ( " Willy - Pete " ) 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 and WP 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 effected 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 gunnery skills of laying ( in a sense , aiming ) the guns . History . The M30 entered service with the U . S . Army in 1951 , replacing the previous M2 107 mm mortar . It was adopted due to the extended range and lethality in comparison to the previous M2 107 mm mortar , although the M30 , at 305 kilograms , was significantly heavier than the 151 kilogram M2 . Due to this heavy weight the mortar was most often mounted in a tracked mortar carrier of the M113 family . This vehicle mounted mortar was crewed by 5 people : the track commander ( mortar sergeant / gun commander ) , gunner , assistant gunner , loader and vehicle driver . Ground mounting of the mortar was time consuming and strenuous as a hole had to be dug for the base plate of the mortar to rest in , sandbags had to filled and placed around the base plate to stabilize it and to protect the exposed ammunition . Also this decreased the accuracy of the weapon as the recoil from firing caused the base plate to shift in the ground . This movement also made the crew have to " lay " the gun back on the aiming stakes more often , causing a temporary lack of fire while the weapon was repositioned and re - sighted back to its original reference point .