Introduced in 1968, the HK33 was Heckler & Koch's adaptation of their roller-delay operating system to the 5.56mm / .223 Remington cartridge. In addition to the standard full-length rifle (with a 15.35 inch barrel) a shorter version was made as the HK33K (with a collapsing stock and 12.4 inch barrel) as well as a "submachine gun" variant, the HK53 (with an 8.3 inch barrel). While it was never adopted by a first-tier global military, substantial numbers were purchased by a number of smaller forces, including Malaysia, Brazil, and Thailand.