Heckler & Koch G11 - Design Details

Design Details

The weapon uses 4.73×33mm caseless ammunition, with the propellant shaped into square blocks. The ammunition has also been designated as 4.92mm because for the HK G11 ACR, a variant developed for US Military trials, the US convention of groove to groove measurements of the bore was employed, rather than land-land. The projectile is 4.93mm in diameter with a case length of 33mm, the US case length measurement is 34mm since for the ACR trials the chamber length, not the actual case length was used. The 4.73mm round is half the weight and 40% the size by volume of the 5.56x45mm NATO round. The round also has similar ballistics to a 5.56x45mm NATO round, but the 4.73mm is much less likely to tumble when hitting or penetrating a target, and thus not as lethal. It is not clear to what extent the round would fragment, which could also make it more lethal. (See terminal ballistics)

The design principle was to increase target hit probability by firing high rate multi-round bursts (salvos). Tests have been run using a prototype shotgun test-bed called CAWS to see whether a single-shot, multi-projectile system could achieve the range and hit probability requirements. The results indicated that the use of serially fired projectiles at a high rate of fire would achieve a tight shotgun-like pattern with rifle-like accuracy up to the required range.

The weapon itself has three firing modes: semi-auto, full-auto at 460 rounds per minute, and three-round burst at over 2000 cyclic rounds per minute, or approximately 36 rounds per second. The loading and feed mechanism is physically very complicated but exceptionally fast and reliable. Rounds are fed into the weapon from a magazine that lies above and parallel with the barrel. The rounds are oriented vertically (at 90 degrees to the bore) and are fed downwards into the rotary chamber so that they can be rotated 90 degrees for firing. The firing cycle process is roughly:

1. As the cocking handle on the side is rotated clockwise by the weapon operator:
2. A round is dropped into the revolving chamber vertically (a loading piston assists this process).
3. The chamber rotates 90° until it is lined up with the barrel. This completes the chambering of the round and cocking of the firing pin.
4. When the trigger is pulled, a firing pin ignites the primer, which then ignites a powder booster charge that pushes the bullet into the barrel. The solid block of propellant is broken up to increase the ignition surface area and ignites, accelerating the bullet out of the barrel.
5. As the projectile is accelerating up the barrel, recoil forces drive the barrel, magazine, chamber and operating mechanism rearwards within the weapon, dissipating energy for single shot and fully automatic modes but allowing burst mode to deliver three projectiles downrange before buffering occurs.
6. Gas tapped off from the barrel rotates the chamber and actuates the loading mechanism then rotating the chamber back to the vertical original position until it is lined up with the feed mechanism and the process repeats.

A conventional assault rifle has approximately eight steps in its cycle:

1. Battery: bolt group pushes round from magazine into chamber.
2. Lockup: bolt or bolt carrier locks with the barrel extension or receiver.
3. Firing: firing pin or striker impacts primer igniting the main propellant charge.
4. Unlocking: either through gas, recoil or blow-back operation, the working parts unlock from the barrel extension or receiver.
5. Extraction: spent case is extracted and withdrawn from the chamber.
6. Ejection: the spent case is thrown clear of the weapon either via a bolt face ejector or from a fixed or semi-fixed ejector.
7. Firing mechanism reset: as part of the rearward reciprocation of the working parts, the firing mechanism is reset.
8. Buffering: working parts finally strike the buffer and halt. Recoil spring(s) are fully compressed and begin to drive the working parts forward into battery.

Because the G11 uses caseless ammunition there are no extraction and ejection steps. Even though the rotary chamber doesn't, in the true sense of the word, lock up, the fact that it has to rotate in and out of alignment with the barrel, the G11 can be considered to have a lock/unlock phase. If a round fails to fire or the weapon is being used with training rounds, the rifle can be manually unloaded by twisting the cocking handle counterclockwise. This pushes the failed/training round out an emergency ejection port on the bottom of the rifle and loads the next round.

The recoil in the three-round burst is not felt by the weapon's user until after the third round has left the chamber. This is accomplished by having the barrel and feeding mechanism "float" within the rifle casing. When the rounds are fired, the barrel, magazine, chamber and operating mechanism recoils back against recoil springs several inches. Only when it strikes the buffer at the back of the rifle does the user feel the recoil. During the rearward travel of the internal mechanism the rifle loads and fires 3 rounds. When the barrel and mechanism reaches the rearmost point in its travel, the recoil springs push it forward back into its normal forward position. When firing in semi-auto and full-auto modes, the rifle only loads and fires one round per movement of the internal mechanism. Fully automatic fire is reduced to around 460 rounds per minute. The internal workings of the rifle were rather complex compared to those of some earlier designs, with the guts being compared to the inside of a compact clock. The number of hours of maintenance required for the G11 as compared to other designs is not clear, especially since the impact of the powder used in the caseless ammo remains unknown. Designers claimed that because there was no ejection cycle the internal mechanisms would have little chance to get exposed to external dust, dirt and sand and this would supposedly reduce the need for cleaning.

There were reports that due to the high tolerances required to seal off the front and rear chamber openings the expected life of the contacting parts was around 6000 rounds before maintenance was required.