The Contender: HK's 40mm GMG

by Dan Shea
Editor-in-Chief / Technical Editor, The Small Arms Review

During a discussion at the ADPA show In June, the HK team suggested that this writer come out to their newest test of the 40mm GMG, a select-fire grenade machine gun. Alluding to a "torture test," I never dreamed that they also had a torture test of the participants in mind. Yuma, Arizona in August is consistently above 110 degrees Farenheit. So, camera and notebook in hand, I put on my desert hat, dropped into Pheonix, grabbed a car, and made ready for the appointed day -

Ever since the invention of explosives, men of war have been looking for new ways to throw "bombs" accurately from a distance. History is filled with tales of cannon, catapult, mortar, sappers and grenadiers, and many of the stories are of the mishaps involved. Avoiding "short rounds," "fast fuses" and "tube bursts" has been a major preoccupation of those who design and use projected explosives. Mechanical fuses were an innovation that solved many of the problems, and the newer spin-armed types have allowed more security to the operators.

Modern armies have many types of weapons at hand. The grenadier of the 1960s had a dedicated weapon, the M79. In order to make every grenadier a rifleman as well (and vice versa), the M203 series of launcher was developed. In a parallel development, Honeywell worked on a crank-operated 40mm launcher for tripod or patrol boat mount using the same low-pressure round as the M79. Then there was a helicopter pod-mounted unit that fired a newer, longer 40mm x 53 round. This new round had a much longer range than the old one, and it had too much recoil to be used as a shoulder-fired weapon. Ranges of 2000 meters were recorded. The U.S. Naval Ordnance project with Colonel George Chinn produced the MK 19 series of fully automatic grenade machine guns, and this fine weapon is still in use today, manufactured as the MK 19 Mod 3 by Saco Defense Inc. (See MGN Vol. 8 No. 12, May 1995.)

There is an old expression "close only counts in horseshoes and hand grenades," which applies to most dual-purpose HE rounds. Machine guns were originally intended as "area" weapons, not "point" weapons. The art of machine gunning has suffered greatly with the recent emphasis on "point" use in the military, which can be traced to doctrine that is geared more for a belt-fed rifle use than a squad automatic weapon. With the introduction of the fully automatic grenade machine gun, and their extended ranges, we give new meaning to the term "area" use.

A grenadier needs to get his grenades within a certain distance of soldiers (soft targets) in order to inflict casualties. He must also hit directly on vehicles or bunkers (hard targets) when the shaped charge aspect of the ammunition is required to penetrate armor. It was with these two thoughts in mind that this individual participated in the Yuma tests. I wanted to find out if Heckler and Koch's newest weapon could perform in both of these diverse roles while standing up to the desert environment.

MGN photographer Jim Bonis, gunsmith Stan Andrewski and I had torn the 40mm GMG down to its basic parts at the ADPA show, so I had some familiarity with the weapon. Heading out to Yuma to go one-on-one with the actual designers was a special treat; unfortunately, security only allowed one person to go. (Guess who drew the "short" straw. Being Technical Editor has its benefits.)

The 40mm GMG was developed at HK GmbH over the last four years, and the one we tested was prototype serial number 011. It is a blowback-operated gun. It uses the same feed system as that on the MG42 and fires from an open breech. The round is stripped from the belt and moved forward into the chamber. Because of the method of feeding, the belts of ammunition must be reversed from the U.S. MK19 use - the male end of the link must be presented first. HK has designed a link that will work in both grenade launchers, and it is waiting for approval from the manufacturers of ammunition as well as a blessing from the other end users. Until that point is realized, and all of the ammunition is standardized, every can that gets opened must have the belt pulled and reinstalled in reverse.

The Germans had scheduled the test at Yuma Proving Grounds on the Mexican border in Arizona, where the U.S. Army has extensive testing facilities. HK had hired the YPG as a sight for their final environmental testing to meet the German Army's criteria. Winter testing is done in Northem Europe, of course, but the best desert facility in the world is at Yuma. Hot, dry, and dusty, YPG is able to assist military testers in actually putting their products through controlled desert testing, not merely a simulation. (Unfortunately the testers also achieve the same reality check. In the 116 degree fahrenheit heat, we grilled lunch on a wood fire out in the desert - I was thinking about cold cuts and ice water, but the hot chicken was excellent!)

Ten thousand rounds of exploding target practice ammo needed to be fired in order to complete the test - at $38 per round, that adds up fairly quickly. I personally fired over 300 rounds. Ranges varied from 500 to 1500 meters. By choice, I did most of my firing at 1000 meters. As much fun as this sounds like, if you ask any "tester" he will tell you the same thing. After a hundred rounds or so, it becomes work - and this was serious work. HK needed answers on how well their 40 mm GMG would work.

At 1,000 meters the 10' x 10' plywood target looked extremely small. It was the kind of target you might use to sight a scoped rifle to center, then check the group and start getting serious on smaller targets. After my first couple of rounds dropped short, I got the feel of it and started seeing white flashes on target every time! This was pretty impressive - first time on the launcher and dropping them right in! Volker Kurtz, from HK GmbH remarked that it was a wonderful thing that a novice such as myself, who had little experience with machine guns and grenade launchers, could learn so quickly ... but, honestly, this weapon was right on target. Almost every shot I fired from it, using the reflex sight, went right into the kill zone. With some more practice, I was pretty confident that I could place rounds consistently inside a normal window-sized area. That's not a brag; I was really impressed with the accuracy of the 40mm GMG. At 1000 yards it was accurate; at 500 yards it was absolutely zeroed!

Area fire was a little trickier. Every automatic weapon has a natural cyclic rate - the recoil of the gun, its return, and the bolt travel, combined with the effect on the operator. With a grenade launcher firing sustained fire (it's not usually referred to as automatic on these launchers), being out of sync with that rate can have a drastic effect on where the grenades land and the size of the group. We do not speak of 4 inch groups at 800 meters. We think more in terms of 16 meter groups. Once I had figured out how to "ride" the 40mm GMG, I was very impressed with how tight the groups were. I could pick an area of brush on the 750 meter mark and cover it with a 3-second burst. I was done before the first rounds hit and could sit back and watch the bright flashes of light saturate the area. These groups were pretty impressive. Again, the objective is to "cover" an area, not put them all in the same spot. Keeping the group fairly tight was easy. Indirect fire with a good spotter would be better than any mortar could dream of being.

The 40mm GMG is significantly lighter than the MK 19 Mod 3. At 29 kg., it is 7.5 kg. lighter. As long as the strength and accuracy is there, lighter is usually better. One of the methods of lightening the design is through the simplicity of design and manufacture. The receiver is cut from a one-piece aluminum extrusion instead of being made from welded steel plates. The bolt is much simpler than the MK 19 as well.

The ease of switching from a left feed to a right feed is illustrated in the photo sequence [omitted here], but the 40mm GMG always ejects to the right. Disassembly can be done (and was with me as a witness) in about 1 1/2 minutes. Putting it back together was just as swift, and no tools were used other than some of the pins that were taken from the 40mm GMG as part of disassembly.

There were two methods of firing the unit when it was tripod-mounted. The operator can choose to use the spade grips and a thumb trigger, which is similar to the M2HBs, or to use the "motorcycle" type grips that were on the soft mount. I preferred the "motorcycle" grips in both of the mounts that were available for us to use. The ground mount was a Norwegian lightweight aluminum unit that worked quite well when extended properly. The first time I saw it used there was a lot of "flex" to it, but it had not been properly secured. The other mount was firing from the Humvee. This was a much more secure mount, and dialing in fire with the traverse and elevation mechanism was a cinch - with the reflex sight it was like watching TV! The 40mm GMG will mount anywhere that the M2HB or MK 19 will. Safety features are important on most firearms; on a fully automatic grenade launcher they are absolutely critical. Imagine a round stuck in the bore, and another round strikes it from behind. Even considering the spin arm fuze, you have more than a "belled" barrel. HK built in a number of safety features:

Safety/Fire Selector Lever

The safety/fire selector lever features the settings "Safe," "Single Fire" and "Sustained Fire." Setting the safety/fire selector lever at "Safe" not only blocks the trigger mechanism but additionally locks the bolt in its cocked position.

Loading Safety

By opening the feed cover, the open bolt will automatically be locked in position, protecting the operator from injuries from an unintentionally-released and forward-snapping bolt.

Firing Pin Safety

This independent safety system ensures that in any event the firing pin can only reach the primer after the cartridge has been chambered so far that the cartridge case is fully supported by the chamber.

Bolt Lock

This mechanically locks the bolt in position and permits the operator to clear the Grenade Machine Gun safely without fear of bolt movement.

Cocking Safety

In case the gunner fails to pull back the bolt completely and releases the cocking handle before the bolt has been engaged in its cocked position, an integral automatic mechanism prevents the bolt from traveling forward and accidentally firing.

All things considered, the HK 40mm GMG is now "the contender"; no question about it. Saco Defense is rumored to have a new lightweight MK 19, but this author has not had a chance to look it over. The lightweight 40mm GMG is an accurate, effective, well-designed weapon. The innovation of design should help to keep the manufacturing costs down and make this a very competitive design.

Note: HK does not want any contact from non-military or unqualified end-users regarding the 40mm GMG. Military Procurement may contact: Wayne Weber, HK Inc. Sterling, VA. (703) 450-1900.

GMG Technical Specifications
Caliber 40mm x 53
Modes of fire Single and continuous fire
Ammunition feed Pawl-operated links
Rate of fire 330 rpm
Maximum range 2,200 m
Muzzle velocity 241 m/s
Weapon weight 29 kg
Tripod weight (with cradle soft mount) 19.5 kg
Barrel length 584mm
Overall length 1,100mm
Width 140mm
Height 208mm
Sights Optronic sights, night sights and back-up iron sights

Originally published in the Novemver 1996 edition of Machine Gun News