I design primarily for the Interwar and WWII periods, and in 1:100 scale for 15mm gaming.
These days I print models almost exclusively in resin, but wherever it is feasible I design with tolerances suitable for FDM as well.
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.