Later in WWII the need for a shorter, lighter rifle for use in the jungles of the Far East led to the development of the Rifle, No. 5 Mk I (the "Jungle Carbine") With a severely cut-down stock, a prominent flash hider, and a receiver machined to remove all unnecessary metal, the No. 5 was both shorter and 2 lb (0.9 kg) lighter. Despite a rubber butt-pad, the .303 round produced too much recoil for the No. 5 to be suitable for general issue. Production of the No. 5 Mk I ceased in 1947 due to an "inherent fault in the design", often said to be a "wandering zero" and accuracy problems. However, the No. 5 Mk I was popular with soldiers owing to its light weight, portability, and shorter overall length than a standard Lee-Enfield rifle.

An Australian experimental version of Jungle Carbine, designated Rifle, No. 6, Mk I was also developed, using an SMLE MK III* as a starting point (as opposed to the No. 4 Mk I used to develop the No. 5 Mk I Jungle Carbine). The No. 6 Mk I never entered full production, and examples today are extremely rare and valuable to collectors. A "Shortened and Lightened" version of the SMLE Mk III* rifle was also trialled by the Australian military, and a very small number were manufactured at SAF Lithgow during the course of WWII.