Mind to Machine writes:
This shotgun is a one of the simplest firearms that can be built. That said, this is for educational purposes only. Do not attempt to build this gun! The materials and ammunition are cheap and commonly available, and it can be made quickly with a minimum of tools.
Here is a live demo of it in action:
It utilizes the so-called “slam-bang”, or blow forward, action. To fire, a 12 gauge shotgun shell is placed into the barrel, which is then slid into the receiver, and slammed rearward. A nail crushes the primer in the process. After firing, the barrel is removed, and the spent shell pulled out by hand.
It is made from two pieces of steel plumbing pipe and an endcap: one pipe acts as the barrel, the other, larger diameter pipe and endcap as a receiver. A small piece of wooden dowel and a nail make up the hammer and firing pin. The stock is made from a length of ordinary 2×6 lumber (I think this is probably spruce).
To comply with US Federal Title 1 requirements: It has a barrel length of 19″ (18″ federal minimum) and an overall length of 35″ (26″ federal minimum).
The body of the receiver is made from a 1″ diameter, 6″ long, schedule 40 galvanized pipe nipple. This comes threaded on both ends. I cut off the threaded section on one end, this will be the front of the receiver that accepts the barrel. The threads on the other end will be used to attach a 1″ pipe cap later. Two holes are drilled and tapped on the bottom to attach to the stock, and a hole is drilled and tapped on the side to provide the barrel lock/safety bolt.
The most critical task for building the receiver is that the barrel freely slides all the way inside of it without sticking or binding. As purchased the two will not likely fit do to manufacturing tolerances and imperfections – the dimensions are very close. Either the barrel outside can be made smaller via grinding or sanding, or the receiver inside can be made bigger via sanding. Or, some combination of both.This is what I did.
One trick is to find a 1″ pipe nipple that has no internal weld seam. Plumbing pipe of this kind is made by bending a flat piece of steel over a cylindrical mandrel and welding the two ends together to form the finished pipe. This creates a weld seam that is invisible on the outside but usually very obvious on the inside. This seam sticks out and interferes with the barrel’s insertion. I have discovered that some pipe nipples have this weld bead ground off already inside, and some do not. It is possible to remove this seam oneself but it is time and labor intensive, and requires making a tool. The tool is a wooden dowel with coarse sandpaper glued to the outside.
It is much easier to slightly reduce the diameter of the barrel since it can be reached with tools. However care must be take not to make it too thin. This is covered below in the barrel section.
The Hammer/Firing Pin
The hammer is fixed at the rear of the receiver and relies on the barrel to bring the shell to it. It consist of a 1″ diameter oak dowel that is cut to 1/2″ in length. The firing pin is made from a roofing nail that is cut to 5/8″ long with hacksaw. The tip of the nail is rounded over with a file to give a blunt profile. It is important not to use a sharp tip as it may pierce the primer instead of crushing it. A hole is drilled through the center of the dowel using a drill bit the same size as the roofing nail. Lastly, a piece of cardboard is cut to 1/8″ larger in all directions than the oak dowel – this serves to keep the hammer and firing pin secure at the end of the receiver.
To assemble, the nail is punched through the center of the cardboard piece and fed through the hole in the dowel. The tip of the nail should slightly protrude above the dowel.
Then the hammer/firing pin are placed into the threaded endcap. The cardboard piece sticks into the threads of the end cap, preventing the hammer from falling out.
If if needs to be removed for replacement or cleaning a pair of needle-nose pliers works fine.
The barrel is a 19 inch long peice of 3/4-inch schedule 40 galvanized steel pipe. Black pipe could also be used here. If the steel pipe has threads they should be cut off on the breech side otherwise the chamber will be weak. This can be accomplished with a hacksaw. The barrel should not be shorter than 18″ to comply with US federal legal requirements. 18.5″ is a good minimum to observe. I made sure to square off the chamber end with a file so the lip of the shotgun shell would rest on the edge snugly.
I made a slant compensator in the barrel by cutting the muzzle at a 45 degree angle with a hacksaw, then cleaning the cut up with a file. This in theory will reduce muzzle flip. An optional front bead sight was also made using a machine screw to give something to aim with. I drilled a hole and tapped for the threads, in this case 10-32. The screw is ground on the bottom to prevent it from protruding into and potentially obstructing the barrel, a dangerous condition for any firearm.
I polished/sanded the barrel near the breach end with a flap disk on a grinder until it slid easily into the receiver without sticking or binding. The trick here was to keep the grinder moving up and down the barrel and never remaining on one spot as you slowly move around the barrel.
I went very easy on the barrel, taking off a very small amount of material all the way around. Shotgun shells operate at relatively low pressure as far as firearms go. But his pipe is not rated for this kind of application. This design however sleeves barrel in another steel pipe during firing to give a some measure of safety.
Making the Stock
I made a basic stock design in MS Paint, scaled it up, and printed on two sheets of paper. This can all be done in the program itself. I then glued the sheets to a piece of cardboard and cut out them out. This created a template for tracing onto the 2×6. I found the stock a tad short and thin for my liking so I slightly changed the dimensions as I traced it to make everything bigger and longer.
The stock is cut out using a panel saw a section at the time. I shaped the edge of the stock using a router with a round-over bit. This could also be accomplished with 60 or 80 grit sandpaper, but much more slowly.
The stock is inlet for the receiver using a hole saw that is slightly smaller than the receiver. The next step was to wrap some sandpaper around the receiver and use it as a sanding block to bring the bed to correct size. I had to be careful to prevent the edges of the inlet area from cracking.
Once complete, I lined the bed of the receiver with some fiberglass tape and wood glue to reinforce the relatively fragile edges. The receiver was wrapped in aluminum foil as a release and clamped down onto the glue. If I were to do it again, I would used epoxy here instead. The glue isn’t really ideal for bedding a stock (too soft), but it works well enough in a pinch.
3/8″ holes are drilled to accommodate the two 3/8 bolts which will hold the receiver to the stock. These bolts are way overkill but they are what I had flying around. Much smaller bolts could be used. The stock has a wooden block in front of the endcap to provide some measure of safety to the user in the event the receiver end threads should fail – the end cap would not be blown directly into the user’s face. This will be later reinforced with screws as it is not very strong – the grain of the wood runs front to back.
The bottom of the holes are counterbored with a big drill bit to recess the bolt heads. They must be tightened with a socket wrench.
The receiver was previously drilled and threads tapped in the holes. The bolts only engage the threads for the thickness of the receiver(maybe 1/8″), as the barrel needs to freely slide past them.
A few washers were added under the bolt heads to get the bolts to tighten up without going too far into the receiver and blocking the movement of the barrel.
The receiver was then removed from the stock, and sanded lightly by hand. Two coats of gunstock stain are wiped on with a rag, with 4 hours between the coats.
Then three coats of polyurethane are applied with a very light scuff sanding between coats. 400 grit sandpaper was used and very little pressure was applied. A sanding block is used to keep from sanding through each coat.
A rag was used to apply the poly. Several companies make “wipe on poly” specifically but the regular stuff worked just fine. At least 4 hours were given between the coats, and a fan was used to speed drying.
I placed the butt of the stock on the flip flop and traced with a permanent marker. I cut it slightly larger using a pair of scissors.
Next step was to drill some shallow holes in the buttstock to provide a good surface for gluing. Gluing to end-grain does not work well otherwise.
Then I ran aluminum tape around the butt-stock to prevent glue from getting on the finish, and to protect it from sanding of the pad later. I mixed some 5-minute epoxy, slathered it onto the stock, and pressed the recoil pad into place. I used some masking tape to hold tight while it cures.
After the epoxy cured, I sanded the foam with 60-grit sandpaper to get the desired rounded over shape. I also masked off the pad and stock afterwards and painted a 1/4″ black band next to the pad.
This band helps hide my less than stellar sanding job on the pad/stock interface that exposed the raw wood. This would probably be less of a problem if I attached and sanded the pad *before* finishing the stock.
The shotgun is assembled as shown:
The receiver is attached to the stock with the bolts, the endcap screwed and tightened onto the receiver end. The final step is to insert the barrel, and mark through the receiver bolt stop hole with a permanent marker, and then make a divot in the barrel using a drill. This divot allows the barrel to be securely “locked” when the bolt is screwed in. This is important as the barrel would otherwise fall out of the gun when carried.
I made a divot with the barrel fully pressed against the endcap, and then moved it forward half an inch for another divot. This way I can secure the barrel fully rearward (chamber empty), and forward slightly (shell chambered). This functions as somewhat of “safety” though should not really be relied on as such.
One optional but pretty useful feature would be a forward grip that attaches to the barrel. This would make it easier to slide it rearward and to hold the barrel when removing/inserting a shotgun shell. I may put one on in the future.
This is cool. Another way of making one like the above: