Two of my favourite modelling subjects, the British Power Boats 'Whaleback' air-sea rescue launch and the SE5a WWI fighter both carried the lewis machine gun as armament. Curiously I could not find a reasonable representation of a Lewis Gun that was suitable for 3D printing. I did find a detailed drawing on the web, from Savage's the North-American company who made the gun under licence. They made versions for both UK and US ammunition.
The STL model is not heavily detailed but is dimensionally accurate.
The STL below is to at 1:4.75 scale - the scale of the printed out drawing I worked to, which gives a barrel length of 147mm. To adjust for any other scale is easy, simply divide 4.75 by the scale required and use this number (as a percentage) to scale the STL in your slicing program.
Example, I needed two 1:16 scale guns for my ASRL model:
4.75/16 = 0.296875 so in Cura I scaled the STL to 30% of original size.
For 12" (1:6) action figures like Action Man, scale by 79%.
To print the STL without support, place two copies on your build plate and rotate them so each one has a different side of the gun uppermost. Check the z-height of the model, then set the z-position of both copies to half this figure. This will give you two prints which can be glued together along the centreline.
Use a small layer height (e.g. 0.1mm) for a good finish on small sections and consider using a small nozzle (0.1 or 0.2mm) for printing it at small sizes.