Heckler & Koch HK33

A schematic of the Heckler & Koch roller-delayed blowback mechanism

The HK33 is a selective fire weapon with Heckler & Koch's roller-delayed blowback system of operation. It employs a two-piece bolt consisting of a bolt head with a pair of rollers and bolt carrier. Upon firing, the two cylindrical rollers in the bolt head are cammed inward by inclined surfaces of the barrel extension and impart a rearward motion on the locking piece, which also propels the bolt carrier rearward. This built-in mechanical disadvantage delays the movement of the bolt head relative to the bolt head carrier which is withdrawing at significant higher velocity of the bolt head. The rollers soon compress entirely into the bolt head, clearing the locking recesses of the barrel extension, and both parts now continue rearward together, opening the breech and actuating the extraction and feeding cycles. The chamber is opened under high pressure, thus the chamber received a series of flutes in order to increase extraction reliability and prevent sticking of the spent casing to the chamber walls.

The G3 roller-delayed blowback mechanism designed around 7.62×51mm NATO ammunition was downsized and revised for reliably using 5.56×45mm NATO ammunition. This required changing geometrical transmission ratio relationships between parts. Based on the geometric relationship arising from the angles of the roller contact surfaces of the locking piece and the barrel extension recesses, the recoil of the bolt head is delayed by a ratio of 3:1 for the 5.56×45mm NATO chambering. Thus during the same period of time, the bolt head carrier moves 3 times faster than the bolt head. This ratio is continued until the locking rollers have been withdrawn from the barrel extension recesses.

Like the G3 bolt the HK33 bolt features an anti-bounce mechanism that prevents the bolt from bouncing off the barrel's breech surface. The "bolt head locking lever" is a spring-loaded claw mounted on the bolt carrier that grabs the bolt head as the bolt carrier group goes into battery. The lever essentially ratchets into place with friction, providing enough resistance to being re-opened that the bolt carrier does not rebound. The spring-powered claw extractor is also contained inside the bolt while the lever ejector is located inside the trigger housing (actuated by the recoiling bolt). Further like the G3 the G33 also contains a spring extractor and a buffer. The ejector lever was installed in the trigger housing and is actuated by the recoiling bolt.

The HK33 is a modular weapon system. Its butt-stock, fore-stock and pistol-grip/fire-control assembly may be changed at will in a variety of configurations (listed below). Simple push-pins hold the components in place and removing them will allow the user to remove and replace parts rapidly. The rifle is disassembled into the following components for maintenance: the receiver/barrel, stock with return spring, bolt assembly and trigger pack in pistol grip.

The HK33 has a conventional hammer-type firing mechanism. In the standard version, the rifle comes equipped with an ambidextrous trigger group with a selector lever that is simultaneously the weapon's safety (it has three positions: "S" or "0"—weapon is safe, "E"/"1"—semiautomatic fire, "F"/"25"—continuous fire). The "safe" setting mechanically disables the trigger. The trigger groups can be swapped out to meet the user's specific mission requirements. H&K offers several different trigger assemblies: a three-shot burst fire control group with selector lever/safety (selector settings: "0" weapon is safe, "1" single fire, "2" 2-round burst or "3" 3-round burst; the selector lever is ambidextrous); a "Navy" trigger unit (three settings: safe, semi and full auto fire) and a four-position trigger group (selector settings: safe, single fire, 3-round burst and automatic fire).

The rifle is fed from 20- or 25-round steel magazines weighing 250 g or 40-round aluminum magazines (weighing 157 g). 30-round arch magazines were also introduced for use with the rifle. Turkish MKEK-made rifles are issued with 30-round polymer magazines.

The barrel contains 6 right-hand grooves and terminates with a slotted flash suppressor that enables the use of rifle grenades and supports a standard G3-type bayonet that mounts above the barrel. The barrel chamber is fluted, which assists in the initial extraction of a spent cartridge casing (since the breech is opened under very high barrel in internal cartridge case pressure). Initially the rifle was produced with a 305 mm (1:12 in) twist rate barrel, which was later upgraded to the faster 178 mm (1:7 in) twist rate (used to stabilize new, heavier NATO-standard SS109/M855 ammunition).

Drehvisier rotating drum sight pictures

The firearm is equipped with a relatively low iron sight line that consists of a Drehvisier a rotary rear drum and hooded front post. The rear sight is mechanically adjustable for both windage and elevation with the help of tools. This deliberately prevents non-armorers to (re)zero the iron sight line. The rotary drum features an open V-notch (numbered 1) for rapid target acquisition, close range, low light and impaired visibility use and three apertures (numbered 2, 3 and 4) used for: 200–400 metres (219–437 yd) in 100 metres (109 yd) increments for more precise aiming. The 1 V-notch and 2 or 200 metres (219 yd) aperture settings have an identical point of aim. The V-notch and apertures are calibrated for 5.56×45mm NATO ball ammunition.

The receiver housing has recesses that work with STANAG claw mounts/HK clamp adapters (standard with the HK33, G3, G3SG/1 and MP5) used to mount day (typically the Hensoldt 4×24 telescopic sight) or night aiming optics. The Hensoldt Fero 4×24 telescopic sight and mount assembly were developed for designated marksman use. The Fero elevation knob features Bullet Drop Compensation (BDC) settings for 100–600 metres (109–656 yd) in 100 metres (109 yd) increments calibrated for 5.56×45mm NATO ball ammunition.[3]

Included with the rifle are a detachable bipod, bayonet (from the G3), sling, cleaning kit and a magazine loader. Additionally, the HK33 can be used to mount a 40 mm under-barrel HK79 grenade launcher or a blank-firing adaptor.

During its production life the rifle received several minor improvements (these modified weapons are sometimes referred to collectively as the HK33E). The fixed stock was strengthened and the synthetic forearm replaced with a handguard that allows a lightweight bipod to be attached and stowed into two grooves at the base. The shoulder pad in rifles fitted with a telescopic stock was changed to a concave type used thus far in the MP5 series.

The Heckler & Koch HK43 is a semi-automatic rifle based upon the Heckler & Koch HK33 rifle and is the predecessor of the Heckler & Koch HK93.

HK43

Heckler & Koch also manufactured a semi-automatic only variant of the HK33A2 for the civilian market called the Heckler & Koch HK43. The HK40-series was designed for sale to conscripts so they could be familiar with their service rifle before entering military service, a common practice in Germany and Switzerland. They had a bayonet mount and furniture just like the military model, but came with a semi-auto trigger pack instead of a full auto trigger pack. This allowed a civilian rifle to be easily made into an assault rifle just by dropping in a full auto trigger pack. It would be succeeded by the HK93A2 and its retractable stock version the HK93A3. The HK93 series had a redesigned semi-automatic trigger pack and metal shelf installed in the trigger group well that made it impossible for it to fit a full-auto trigger pack. It also had the bayonet mount removed and had different furniture.

C-93

Civilian semi-automatic sporting version produced by Century International Arms, Inc. It comes with an 18.9 or 16.25-inch (413 mm) barrel with a 1:9 twist ratio. A carrying handle and 40-round magazine are standard. Advertised weight is 8.2 lbs. The C-93 is built from Thai Type-11 parts kits using an American made barrel and other miscellaneous American parts.