Since there making the Sten and already made the MP40, why not add 1+1 together to get the M3 Grease Gun. The M3 Grease Gun is an American SMG tht was used in WW2 and even saw service up to the Gulf War. I believe its still in service by NATO countries, but Im not sure. The Grease Gun was designed to replace the Thompson, and was based off the Sten and the MP40, so to me, it makes sense to add this famous WW2 SMG. Sten MK. II+MP40=M3 Grease Gun
. The link above is a disassembley guide I found.