HK 36 Prototype
Not to be mistaken for an early version of the modern G36 rifle, it was invented in the early 1970s and it was one of the first rifles (if not the first) capable of three-round burst fire as well as full-auto. It fired a special low-weight, low-impulse 4.6x36mm "spoon point" ("Loffelspitz" in German) cartridge, with an almost flat trajectory of about 300 meters, which, as I understand, was also its effective range. Furthermore, it had an integral 25-round magazine, which could only be loaded by pulling down the magazine platform and sliding a pre-packed box of cartridges into the open rear of the magazine. The recoil mechanism was H&K's usual roller-locked delayed blowback. Info source: Jane's Guns Recognition Guide book, by Ian Hogg.
Photo and information from: LJ-38M & Pierangelo Tendas