Currently the US army and USMC are deficient in the capability of platoon level indirect fire weapons. Machine guns may be able to pin a foe down at 600m+, but the platoon has no weapon system that can then drop HE onto the immobilized foe. The company mortar section only has two weapons, so mortar support from the company level may not be available, particularly if the platoon is operating independently, as may be necessary for counter-guerrilla operations.
In most armies this need is met by a platoon level mortar usually of either 51mm or 60mm calibre. The 60mm is the more popular option, since it can share ammunition with company level weapons of the same calibre. There are, however, valid objections that the weight and bulk of the individual rounds (around 3¾ lbs) limits the number that a platoon mortar team can carry. The British army (which doesn't use company level 60mm weapons) instead uses a 51mm weapon that fires 2lb rounds. The objection to this system is that the weapon is rather a logistical orphan.
In World War II the Imperial Japanese Army used 50mm Type 10 and Type 89 Knee mortars
Performance and weight of these weapons compares very favorably with the current British systems. Only complication was that the weapons could not use all issue grenades, only Type 10 and Type 91 models that had a propellant cartridge screwed onto their base. There was also a dedicated shell for the Type 89.
I now intend to expand further on this concept with the proposal of a Platoon-level High Angle Grenade Launcher (HAGL). In external appearance this will closely resemble many of the current platoon mortars. There will be a small baseplate and a barrel of between 37mm and 45mm in calibre. There will probably be a carrying handle that includes the trigger system. Between the baseplate and the barrel will be a mechanism for firing propellant cartridges. This will probably be fed by a Bren style top loading magazine. The barrel from this mechanism will extend into the interior of the external barrel. This interior barrel or spigot will have an external diameter of 22mm. At the muzzle of the external barrel will be a structure that flares out then becomes a parallel walled section of approximately 2.5/66mm calibre. This structure is offset so its lower interior surface is continuous with that of the smaller calibre external barrel. This allows rounds to be dropped smoothly into the weapon. The enlarged calibre muzzle assembly has two functions. Firstly, it acts as a blast attenuation device (BAD). The second function is that it can be used as a cup discharger for the launch of conventional hand grenades. Round bottomed grenades such as the M67 will rest in the conical section, while flat bottomed grenades will sit above this section.
Main proposed round for this system would be Rifle-Hand Grenades. These would be loaded like conventional mortar bombs. They would slide down the external barrel and over the spigot, and be fired by the discharge from the propellant cartridge. This cartridge would be more powerful than 5.56mm or 7.62mm rifle rounds, so would project the Rifle Hand grenade at a higher velocity and to a greater distance. Many other forms of dedicated Rifle grenade may also be compatible with this launch system.
In cup discharger mode the weapon can launch any unmodified hand grenade with a nominal diameter of 2.5. This includes the M67, L109A1 and M68 fragmentation grenades and various WP, HC Smoke, thermite and Riot control grenades. The grenade is placed in the cup and the pin removed, the wall of the cup holding the safety lever down firing. Propellant gas leaving the inner barrel fills the chamber of the outer barrel and the pressure acts on the base of the grenade.
Various grenades of oval shape and smaller diameter are still in use by many armies, including versions of the M26, L2A2 and No36M Mills bomb.
These can be quite simply adapted to HAGL launch. The Mills bomb was adapted to launching from the 2.5 EY grenade launcher by a disc that screwed onto the base of the grenade. A modern version might clip on or use adhesive. Another interesting option is shown here. Such an adaptor might form part of the grenade's shipping container. When used as a sabot for launch from the HAGL it would delay the release of the safety level until the grenade and the sabot separated, increasing the time to detonation and the range.
The HAGL will use several sighting modes. Basic system will be a bubble clinometer combined with a line engraved and painted near the muzzle. As a quick aiming system the carrying sling can be marked with various ranges. The Soldier places his foot on the range mark, raises the weapon until the sling is pulled taunt and fires. An electronic sighting system would measure the weapon's angle of inclination and display the estimated horizontal range on a digital display. Changing a setting would allow for the differing ballistics of various rounds.