In the mid to late 1960s, Heckler & Koch developed the HK33, which was a scaled down version of the Heckler & Koch G3, but chambered for 5.56x45mm NATO rather than 7.62x51mm NATO. The HK33 entered production in 1968. In 1974, a semi-automatic version of the HK33 was introduced by H&K and was designated the HK43. According to H&K’s numbering nomenclature, the “4” indicates that the weapon is a paramilitary rifle, and the “3” indicates that the caliber is .223. The HK43 later became the HK93. HK93s were sold with 20 round aluminum magazines, although the original HK33 assault rifle came with 40 round magazines. HK93s will still accept 40 round magazines, though.
The HK93 was, for the most part, identical in appearance to the HK33. H&K also omitted the grenade launching snap rings on the barrel, as they had for their HK93 models, because the Gun Control Act of 1968 prohibited such features on imported rifles. The HK93 was also fitted with an all-plastic MP5 style butt stock with the recoil buffer attached to the bolt carrier. The HK93 is a semi-automatic weapon with Heckler & Koch's signature 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 carrier which is withdrawing at four times the velocity of the bolt. 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 very 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 HK93 has adjustible iron sights, a conventional hammer-type firing mechanism and HK's standard 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 rifle is disassembled into the following components for maintenance: the receiver, stock with return spring, bolt assembly and trigger pack with pistol grip. The barrel is equipped with a slotted flash suppressor that enables the use of rifle grenades and supports a standard G3-type bayonet that mounts above the barrel, which helps the rifle reach its legal length of sixteen inches.