Lewis Gun — 15mm and 28mm
The Lewis gun first appeared in World War One, but it soldiered on well into World War Two in second-line service, having been largely replaced by the Bren. It was used a lot to provide rooftop convoy AA protection for British trucks during the Battle of France, and in the early years of the Desert War.
This zip file contains STLs for a 28mm (1/56) and 15mm (1/100) lewis gun, a pole mount for the 15mm model, and a sprue of six 15mm guns and mounts.
I have had the 15mm sprue successfully printed in resin, but I highly doubt that it would be worth printing in FDM.