Improvised and home made firearms
This topic surfaces from time to time, usually to indicate the futility of outlawing firearms. It is relatively easy to come up with a working firearm if fixed ammunition is at all avilable. Even if not, there is little difficulty in cobbling up some pretty decent black powder for use in muzzle loaders.
News accounts of guerilla and insurrectionist activity in the Philippines and Indonesia especially contain mention of such munitions. Run a Google search for "Improvised firearms Philippines" and you'll get over 30,000 references to check out.
Many "helpful hints" for home made arms center on the shotgun for several reasons. Shotguns are so useful in foraging for food and for short range defense that they may often remain available to oppressed people long after handguns and rifles are outlawed. Even once these are confiscated, individual rounds of ammunition remain in hiding or in circulation for years thereafter.
Most, if not all, standard sporting-type shotshells have straight-sided cases with rims. This makes it easy to scrounge up the proper size pipe or metal tubing to become the barrel, and which will hold the shell in position for firing. A shorter and slightly larger diameter length of pipe can be located which will slide over the rear of the barrel and form the breech. A firing pin is affixed inside a threaded cap which closes the back of the barrel.
With a shell "chambered" in the barrel, the assembly may be slid into the other pipe with sufficient impact so that the pin impinges the shotshell primer and the shell is thereby fired. All else is refinement. Usually the first is to form a butt stock for the breech end and a handle for the barrel, to provide a measure of control and stability. Next would be a mechanism to prevent the barrel sliding the shell head into contact with the firing pin. This could be a hole drilled through the two pieces and a transverse metal pin fitted to prevent movement of either component.
It should be noted that not all metal pipe is suitable for such uses. There have undoubtedly been many serious accidents caused by flawed or thin-walled pipe giving way when the shell fired. While it is easy, and arguably legal to home-make such arms, bearing in mind certain restrictions, it is far from safe unless sufficient precautions are taken.
While black powder muzzle loading arms are relatively free from many restrictions, using fixed ammunition requires consideration of barrel length (18" for smoothbores) and overall length (26") minimums.
Best
Johnny