It is one of the many mid to late, and even early cold-war, British SMG experimental designs.
The British had adopted the Sten gun family of SMGs in haste, and they knew that the weapon would not stay in service forever, and so by 1944 they had laid out the requirements/specifications for a replacement SMG. When the initial expediency was over, the British could then look into newer alternatives, more in-line with what they actually wanted from a SMG, rather than just adapting what they could manufacture at short notice as had been the case with the Sten.
Even before these 1944 requirements, many could already see the writing on the wall, such as George W. Patchett, who was already tinkering with his concept of a weapon as early as after the adoption of the Lanchester SMG. What we got was the Patchett, which evolved into the later Sterling L2 SMGs.
There were other SMG designs in play, developed by other people at around the same time. The “Viper” series of SMGs are but one of the contenders, designed by Derek A. Hutton-Williams.
The Viper SMGs were intended for military police duty in post-war occupied Germany. It was thought that such duties would lend themselves to designs more aimed for quick use rather than conventional use, so one-handed use and hip-fire practicality was in mind. It’s not as bonkers as you might think, though, if you’re expecting remnant nazi activity, and you want to give your drivers and motorcyclists a weapon they could respond quickly with if ambushed (theoretically being able to hold on to and control your vehicle as you fired your gun during the retreat).
As for the use of the MP-40 mag, I guess the idea there is that, again, it’s intended for post-war occupied Germany. MP-40 mags would be in great supply as the country would be disarmed, and theoretically manufacture could be restarted again as well. So, it makes sense this, too.
If I didn’t make it clear, no this was never adopted, it ultimately lost to the Patchett in trials. British military authorities weren’t that impressed with any of the Viper SMGs.




