The Sten MkI had barely been approved for production when the Sten MkII was born. Initially requested to produce a version of the gun suitable for paratroopers, in March 1941 Harold Turpin redesigned the front end of the Sten to have a quickly detachable barrel and a rotating magazine well (for compact storage). This new model was tests in late June and early July, approved for use, and contracts for it were issued in August 1941.

Named the MkII, this model of the Sten would quickly become the standard, and it was ultimately produced by six major factories (with the assistance of hundreds of subcontractors) on three continents to the tune of 2.6 million examples made. In addition to the barrel removal, the new model has a simpler front sight, simpler stock, and a revised bolt locking notch (upward, instead of downward like on the MkI).

Many thanks to the Royal Armouries for allowing me to film and disassemble these submachine guns! The NFC collection there - perhaps the best military small arms collection in Western Europe - is available by appointment to researchers:

https://royalarmouries.org/research/national-firearms-centre/

You can browse the various Armouries collections online here:

https://royalarmouries.org/collection/