The Type 96 and Type 99 Nambu light machine guns were arguably the best LMGs used by any nation during WWII - they were light, handy, accurate, durable, and reliable. Designed by Kijiro Nambu to replace his 1922 Type 11 LMG (which was fed by a unique hopper mechanism using 5-round rifle stripper clips), the Nambus are often mistaken for copies of the Bren gun. In fact, the mechanical operation of the Nambus is quite distinct from the Bren; they simply share an external resemblance.
The Type 96 was adopted in 1936 and was chambered for the 6.5x50SR Japanese cartridge. This relatively light cartridge allowed the gun to be smaller and lighter than many contemporary LMGs. All of the guns were made with rails for detachable optics, and used 30-round box magazines. In 1943 production of the 96 ended, as it was being replaced by the Type 99. The 99 was basically the same gun but chambered for the larger 7.7x58 cartridge.
The biggest difference between the two was the barrel changing mechanism. The Type 96 had a simply lever to throw to release the barrel, where the 99 has a nut to tighten the barrel down to the receiver. In addition, the 99 barrels had to be headspaced to specific guns with thin washers, whereas the 96 barrels were universally interchangeable. This change was made to reduce machining cost on the barrels, although it is often misunderstood today as being some sort of adjustable headspace device.