A snap cap is a "dummy" bullet that allows a person to fire an unloaded rifle without damaging the firing pin. Firing a rifle without live bullets allows a person to practice a solid stance, careful aim, controlled breathing, and a gentle squeeze of the trigger. These skills are essential to good marksmanship and a snap cap makes it possible to practice them without the expense of firing live ammunition.
The bullet on the right is an 8 x 57mm Mauser rifle round. The bullet on the left is a snap cap I made from a spent shell casing, an 8 - 32 brass screw, a portion of a 5/16 inch bolt, an 8 - 32 steel nut, and enough hot glue to fill the brass shell casing.
If you search "DIY snap cap" in a search engine, you find a post about making a snap cap for an odd sized caliber by pushing out the spent primer cap from a brass casing and using epoxy glue to adhere a section of a common pencil eraser. I am using original military surplus ammunition made in 1947. It uses the European style primer cap, which does not push out like the US style cap.
Step 1: My Rifle
This is a photo of my rifle. It is a Yugoslav Mauser. It was made on captured German machinery and is nearly identical to a K-98, except for the national markings. It was made in the years following World War II when most countries in Europe thought there might be another war soon. They produced extra rifles to be kept in warehouses so they would be ready. That war did not come and semi-automatic rifles came into production, rendering the bolt action Mauser obsolete. This rifle is in beautiful, factory new condition after almost 60 years in a warehouse. It was never issued.
When I first wanted a snap cap, I could not find any for this rifle's caliber. In recent months I have found some on the Internet.
Step 2: Drill Through the Primer Cap
Notice the indentation from the firing pin on this spent casing. Drill a hole into the indentation. Make the hole just a bit larger than an 8 - 32 screw. Use a countersink bit to remove burrs on the outside of the casing. (If you use a vise, be careful not to crush or distort the casing. The very end is quite rigid and will not easily crush or distort. Grasp it in the vise. I wrapped a piece of bicycle inner tube around the casing to protect it from gouges and scratches from the vise jaws.)
Step 3: Machine a Standard Nut
The photo shows an 8 - 32 steel nut using an 8 - 32 screw as a mandrel to turn the nut by holding the spinning drill near to a grinding wheel. Not only do you want to remove the corners, but you want to reduce the diameter of the nut until it will just pass through the opening in the shell casing's neck.
I have one of those magnetic tools with a stiff spring shank that allows one to reach into corners and retrieve small steel parts. I placed the machined nut on the end of it, pushed the nut inside the casing, and used the magnetic tool to hold the nut over the primer hole so I could start the brass screw into the nut. It was not difficult at all. Pull the nut against the inside of the shell casing with the screw once the nut is started and turn it almost finger tight. A little looseness is fine. The looseness aids in aligning the screw parallel to a center line running down the shell casing, when the time comes.
Step 4: Add a "bullet"
The finished snap cap will not properly cycle through the rifle's chamber when the bolt is operated unless there is something resembling a bullet in the neck of the casing. If you do your own reloading, you can press a bullet into the neck of the shell casing. For anyone without a die and a reloading press, you can make a satisfactory bullet from a piece of rod. I cut the head from a standard 5/16 inch bolt. Then I chucked it in my drill and held it against a grinding wheel, as with the nut in Step Three. Vibration frequently loosened the grip of the chuck's jaws and I had to stop to align the bolt and tighten the jaws.
Step 5: Fill With Hot Glue
I adjusted the alignment of the 8 - 32 brass screw so it was pointing straight toward the center of the hole in the casing's neck. Then I began to fill the casing with hot glue. When it was full to about the neck I inserted my facsimile bullet. I tried to make certain some hot glue covered the threaded portion inside the casing. I was concerned that the hot glue might have cooled as it was flowing down the inside of the casing, so I lightly played over the outside of the casing with a propane torch. Be careful to try to keep the brass screw in the primer hole straight during all of this. Also watch out that the facsimile bullet does not sink down inside the casing. The casing will be almost too hot to touch with your fingers. Have a couple of pliers nearby.
When the hot glue has cooled completely, use a screwdriver and turn the brass screw out of the casing. This breaks the threads loose from the hot glue. You will probably need this later. Put the screw back into the nut.
The nut inside the casing is nearly as large in diameter as the casing. It acts like a plunger when the firing pin strikes the end of the brass screw. The nut pushes against the solidified hot glue inside the casing. The solidified hot glue cannot move because the shell casing is tapered, especially at the neck of the casing. And, the solidified hot glue has some elasticity similar to hard rubber. Commercial snap caps have a spring to absorb the strike of the firing pin. This snap cap has the elasticity of the solidified hot glue.
Step 6: Make the End of the Screw Neat
Saw the head from the brass screw. Then file what is left of the screw so it is exactly flush with the end of the shell casing. Any extra remainder on the screw will decrease the headspace in your rifle's chamber and the bolt will not close. The rifle will not fire.
Here you can see the bicycle inner tube protecting the shell casing from the vise jaws.
Step 7: Success
In this photo the bolt has picked the snap cap up from the magazine, locked it to the bolt, and is about to push it smoothly into the chamber. This snap cap works quite well.
Step 8: When the Brass Screw Wears Too Much
In time the indentation in the brass screw will have a larger and larger dimple. If the dimple grows to be too large, it will be as if you are firing the rifle without a shell in the chamber and you could damage the firing pin.
The indentation you see in the snap cap (left bullet) is the result of about ten releases of the firing pin. After ten more the indentation was still about the same. I believe the solidified hot glue is doing its job absorbing the shock of the firing pin as it should.
You already know you can turn the screw in the nut's threads. You can back it out, as long as you have a slot for a screwdriver. Place a thin cutting wheel in a Dremel tool and make a shallow slot. It is no problem if you cut into a little of the casing's end. Use the smallest screwdriver that works to back the brass screw out a turn or two. File the impromptu screwdriver slot and the dimple out of the end of the screw. Make it flush with the end of the casing so there is no headspace problem, and you are ready to get more use out of your snap cap. If you completely use up the brass screw, insert a new screw and repeat the process in Step Six. You will be ready to go again.
This snap cap works well with a tapered casing, especially one with a shoulder. Some further thinking would likely be necessary for a bullet with no taper, like a .45 Colt ACP.
Also, it is often recommended that snap caps be painted some bright color so no one confuses them with live ammunition, and, worse yet, vice versa.