So, I've been thinking about this for a while now, and I was watching some videos online it got me thinking. Having seen handmade submachine guns made by the IRA as well as commercially produced carbines; I've noticed a few common things. First is blow-back operation. The absolute SIMPLEST automatic action, it doesn't require anything more than a bolt heavy enough to prevent extraction until pressure has dropped sufficiently to prevent say, the shooter getting hot gas in their face. The Second is the receiver being a very simple shape; Square or round doesn't matter, but in desperate times, a simple steel tube will work. The receiver is what the bolt moves back and forth in and should be large enough to hold a sufficiently heavy bolt to delay extraction sufficiently. Third is generally a lack of wood. In a workshop setting, shaping wood into grips and stocks requires more work than is convenient. When you're using spare parts, there are enough metal bits about that work without much shaping. Fourth, magazines. Designing and building magazines can be difficult, it makes sense to build a weapon around a pre-built magazine. Fifth, if you're going to make it potentially full-auto; it makes sense to have it fire from an open bolt. What this means is that when the weapon is ready to fire; the bolt is at it's most rearward and the chamber is empty. When the trigger is pulled, the bolt travels forward, taking a round from the magazine and placing it in the chamber. It usually uses a non-retracting (fixed) firing pin that immediately strikes the primer and fires the cartridge. What would this look like practically? Well given the "pipe" in the name, a bit of wide steel pipe of some kind. The grip and trigger assembly would likely be similarly, steel piping of some kind, welded together creatively, possibly flattened a bit in the case of the grip. The barrel is pinned and welded to the receiver and is necessarily made of steel. Stub barrel would be about 3 inches, short 6 inches, long 10 inches. The barrel would likely be shrouded in the case of the short or long barrel with a sheet metal tube possibly perforated. Instead of porting, the "ported" barrels would include a simple fore-grip. The "finned" barrel would likely still be finned. Aluminum or copper fins MIGHT, in theory, create a larger cooling surface and heat sink effect which, in theory, prevent the point of aim wandering during sustained fire. However in the "finned" barrel, the fins would largely be hidden by the barrel shroud. Stocks would be composed of metal, wire stocks being most common. The Marksman stock might be wood, the idea being that extra time has been taken to make a stock that provides good accuracy. So, I'm going to try to mess around with Blender and see if I can make something that looks halfway decent. If someone with more 3-d modelling experience than I and wants to work on a realistic pipe gun, though I welcome the help. Send me a message if you're interested