By Anthony G. Williams
The US Army’s programmes for developing two different 25mm grenade launchers - the shoulder-fired XM25 from Alliant Techsystems and the crew-served XM307 ACSW (Advanced Crew-Served Weapon) by General Dynamics - are probably the most ambitious small arms projects in existence. They are meant to revolutionise the effectiveness of small-arms fire by detonating HE/fragmentation grenades directly over the target, thereby greatly increasing the number of casualties, not just of standing men but also those hiding behind cover. Such rounds are known as HEAB (High Explosive Air Burst) or ABM (Air Bursting Munition).
To achieve this requires some sophisticated technology. The XM307 is fitted with a day/night sight with a laser rangefinder, a ballistic computer and a fuze-setter. The gunner first lases the target to establish the range (this can be overridden if the target is at a slightly different distance than the aiming point); the system then takes atmospheric conditions and weapon tilt angle into account before indicating the aiming mark in the sights. The gunner can set the fuze mode for airburst, PDSQ (point detonating super-quick), PD delayed-action or deactivate; it also contains a self-destruct element. The projectile fuze measures the distance by counting the number of rotations: its spin rate is 21,000-28,000 rpm. The XM25 uses the same technology, with a shorter, lower-velocity cartridge limiting the range to around 700m rather than 2,000m.
The XM307 was originally intended to replace most of the .50 M2HB and 40mm Mk 19 AGLs, while the nearest comparators to the XM25 are the much bulkier six-shot revolvers chambered for the low-velocity 40mm grenade round, such as the Milkor MGL, adopted by the USMC as the M32. Adoption of the XM25 might therefore be expected to lead to a reduction in the use of the 40mm LV (low velocity) weapons, if not their eventual disappearance.
The manufacturers of 40mm GLs and ammunition are not taking this lying down, however, and have come up with a range of new developments to keep their products attractive.
Ammunition for Shoulder-Fired Grenade Launchers
The 40x46SR low velocity (LV) grenade round has now been around for almost half a century, originally chambered in stand-alone single-shot guns like the M79, but subsequently in underbarrel grenade launchers (UGL) such as the M203, and more recently the six-barrel revolvers mentioned above. The muzzle velocity is only 250 fps, limiting the maximum range to about 400m.
The original round fired was the M406 HE/fragmentation type, and High Explosive Dual Purpose (HEDP) rounds have also been available for some time, enabling these launchers to be used to tackle light armour. Some recent developments have considerably extended the versatility of this round. From Arcus of Bulgaria comes the AR476 “Anti-Diver” grenade which has a time fuze which is set on impact with the water, detonating the HE at a stated depth of between 5 and 12 metres and with a claimed lethal radius of 12m. They have also introduced the AR466 “Bouncing” ABHE grenade which on impact is kicked back up into the air by a small nose charge before detonating, to increase the lethal radius - not a new idea, but a very much cheaper way of providing some of the effectiveness of the sophisticated HEAB systems. A similar RLV-FJ “Jump” round is offered by Arsenal, also of Bulgaria - a country which appears to be innovating with enthusiasm in this field.
More effective conventional ammunition has also been developed, with the latest trend being Medium Velocity (MV) rounds, with the muzzle velocity increased to between 350 and 410 fps, thereby doubling the effective range. Recoil is claimed to be similar to that of a 12 gauge shotgun, and well below that of a rifle grenade, and they are designed to be used in many existing LV systems. At a range of 150m the mid-range trajectory will be halved to about 3m and flight time is reduced. The first in the field was Martin Electronics’ “Mercury”, which is significantly longer than standard 40mm HE rounds to make room for a larger HE charge, doubling the destructive power as well as the range of the conventional M433 HE grenade. Nammo Raufoss, Heckler & Koch and now Denel of South Africa are also developing MV ammunition, while Arcus have developed “extended range” versions of their HE and ABHE products, capable of about 600m.
IMI and STK are even developing High Explosive Air Burst LV rounds to deliver the same effects as the XM25, although these do of course need to be used in conjunction with special sights with a laser rangefinder and fire control computer, and also a launcher modified to include an electronic fuze setter. IMI have developed the compact and lightweight Orion sight which offers these facilities and can be fitted to any rifle in conjunction with a modified UGL.
Martin Electronics have also introduced a thermobaric round, the DRACO; a type of explosive which is proving particularly popular in some larger weapons in use in the Middle East because of the effectiveness of its high blast overpressure in enclosed spaces such as buildings and caves. The round is very expensive, however, so attention is now being focused on the much cheaper Hellhound.
The peacekeeping roles which armies frequently have to adopt have generated growing interest in Less-Lethal ammunition for riot control and similar policing activities. Many manufacturers now offer a very varied range of irritant chemical, impact, combined and paint marker ammunition too extensive to describe here. Although such munitions have traditionally been used in specialised 37/38mm riot guns, they are increasingly available for the 40mm LV grenade launchers - and, in a few instances, for the 40mm HV AGL as well.
Other types of rounds which are not directly lethal include various smoke and flare types. There are also some novel developments, most notably the observation rounds containing parachute-borne video cameras that send pictures directly back to the operator, providing an instant view of what lies over the hill or around the corner. Both STK (with the SPARCS) and Martin Electronics (with the HUNTIR) have developed such rounds for 40mm LV grenade launchers, while the Rafael Firefly, still in development, is equipped with folding “wings” rather than a parachute.
Many of the new rounds are longer than standard, at up to 5.5 inches. In contrast, ammunition for the semiautomatic XM25 is limited in size (the maximum length is only about 3.5 inches), and cannot hope to emulate the variety of types now available for the 40mm LV GLs, which will be with us for the foreseeable future.
40mm AGLs
The success of the original 40mm LV grenade rounds rapidly led to the development of fast-firing weapons chambered for them, but almost as quickly the need for greater range was realised, so the 40x53SR high velocity (HV) cartridge was developed. This had the muzzle velocity increased to around 800 fps which, in conjunction with a heavier grenade (about 240g compared with 180g), increased the effective range out to 2,000m. The first automatic grenade launchers (AGLs - also known as grenade machine guns or GMGs) chambered for this round were externally powered and designed for installation in helicopters: the M75 and M129 both seeing service in Vietnam. Far more important, however, was a self-powered design, the Mk 19. This was developed in the late 1960s as a USN project but was subsequently adopted by other services, as well as achieving substantial export sales.
Since then, and particularly since the 1990s, a number of rival AGLs have emerged from several different countries: the Spanish Santa Barbara (now General Dynamics Santa Barbara Sistemas) LAG 40 SB, the Singapore Technologies Kinetics (STK) 40AGL and Light Weight AGL, the Heckler & Koch HK40 GMG (recently bought by the British Army), South Africa’s Denel Y3 AGL, and other weapons from Romania, Poland, Turkey and Pakistan. Of most significance to the USA is the CG40, a joint project between three companies, commencing in 1995: Nammo (ammunition), General Dynamics Canada (Fire Control) and Saco Defense (now a part of GD) who developed the gun and dealt with system integration. Some 400 are in service with US special forces, ordered between 2001-5, under the designation Mk 47 Striker.
The HV ammunition used in these guns has not shown the variety found in the LV rounds, partly because of the narrower range of uses of the AGLs and partly because the need to function in an automatic mechanism restricts the characteristics - and particularly the overall length - of the cartridge. However, a great deal of attention is now being paid to the development of the same kind of airburst technology as that being tested in the XM307.
The way is being led by Nammo Raufoss, who have developed HEAB ammunition as a part of the CG40 project, under the designation PPHE (Programmable Pre-fragmented HE). The gun was designed for this from the start, the necessary systems being integrated into the weapon. These include an image-intensifying video sight linked to a laser rangefinder and incorporating a ballistic computer to indicate the exact aiming point, and an electronic fuze setter by the gun breech. When used with the appropriate ammunition fitted with the Mk 438 programmable time fuze, the system sets the appropriate time delay on the fuze while it is in the chamber (the gun fires from a closed bolt) so that the grenade bursts directly over the target. The gun can also use conventional point-detonating 40x53SR ammunition, with which the advanced sights are still useful in providing a high level of accuracy. The Mk 285 HEAB projectile produces fragments which are mainly distributed sideways and backwards to catch troops behind cover, rather than forwards as with a conventional grenade. An initial order for 39,000 of this round has been placed for use in the Mk 47 Striker, with deliveries expected to be completed by early 2008.
Nammo are also working on a variation of this system for guns, such as the HK40 GMG, which fire from an open bolt. The chosen system is radio frequency programming, the fuze being set about 4 to 5 metres after the grenade has left the muzzle. Since all of the system requirements are contained within a single sight/fire-control/programming unit, this can be fitted to any gun able to mount the unit.
There is yet a third HEAB system, from STK of Singapore; the ABMS (Air Bursting Munition System), which can also be fitted to existing 40mm AGLs. This was developed in conjunction with Oerlikon-Contraves Pyrotec AG of Switzerland, who adapted the technology developed for their 35mm AHEAD cannon ammunition. As with AHEAD, the fuze-setting system is attached to the muzzle and sets the fuze as the grenade leaves the barrel. Like the Nammo systems, special grenades are used with an all-round fragmentation pattern, so they throw fragments behind them as well as in front and to the sides; the Oerlikon/STK type contains 330+ tungsten balls, each weighing 0.25g and intended to penetrate at least some grades of body armour. The effective range of the system is 40 to 1,600m.
In the rush to airburst systems, the value of the conventional (and much cheaper) ammunition has not been forgotten. Nammo have developed a ‘product improved’ M430 HEDP round at the request of the USMC, who have asked for a mechanical self-destruct fuze, a modified (NICO type) propulsion system, insensitive munition (IM) properties using a PBXN-11 main charge, additional incendiary after-armour effect, penetration improved from three to four inches of RHA, and a reduced safety zone to prevent the base of projectile from being blown back towards the gunner (the current limitations being 310m in peacetime, 75m in wartime). Internal Nammo qualification of this round is expected in spring 2008.
While the 25mm XM307 offers certain advantages, notably gun and ammunition weight plus a much flatter trajectory and shorter flight time, the 40mm AGLs clearly have a lot of life in them yet. Their ability to fire inexpensive standard ammunition as well as sophisticated HEAB rounds means that they are likely to remain popular long after the XM307 enters service.
(Anthony G Williams is Co-editor of Jane’s Ammunition Handbook, and maintains a website at http://www.quarry.nildram.co.uk)
This article first appeared in Small Arms Review V12N3 (December 2008) |
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