Not a scratch build at all, more a discovery of a material that might be of use to some.
A while back I asked whether anyone knew a way to convert .50cal MG's into .30cal M1917 of Vickers MMG. I got one suggestion (insulation stripped from wire) but found it a bit difficult to work with. Well, with Nazgul's recent questions re his helicopter projects and me casually suggesting hollow brass section, I thought I should back up my suggestion with some research, and found something called microtube. I found a model shop within 5 minutes of where I live that actually stocks it, so bought some. I bought some with dimensions of 0.8mm OD and 0.6mm ID. I also bought some at 0.5/0.3. It cost me $12 Aus for 3 lengths of 30cm (a foot).
What I did was then chop off the ammo box of a .50cal (the M1917 might not have a box ammo feed, being canvas belt), and the Vickers .303 feeds from the other side; then trim 1mm off the length of the barrel to bring it to the right length. After that, cut a 1.5mm or so length of the 0.8mm tube and slide an authentic brass water jacket over the barrel of the MG!
Here it is, ammo box put on the right side for a Vickers, mounted to a CinC universal carrier.
Side view to get the profile...
Oh, also makes great gun barrels, mortar barrels!