Here is the headphones that I've modified to be awesome.

For this build I was using:

2 bullet shells S&W .40 Cal
old headphones. It happens that mine fit perfect to the bullet shells without any glue. I was using TDK EB760
10mm wooden dowel
sandpaper 400Grit and 800Grit
finishing sponge (You can replace it with high grit sandpaper or steel wool)
a hacksaw
a battery powered drill
a drill bit (diameter of the drill bit should match the diameter of the wire)
a soldering iron
a vice or a clamp to fix the bullet casing for drilling

Step 1: Drilling a Hole

To drill the hole for a wire you need to fix the shell in a vice or clamp it to a table.

Mark the future hole using a centre punch or a nail. Then drill the hole. Brass is a very soft metal so it should be very easy to drill.

Step 2: Cleaning Up the Shell

Clamp the 10mm wooden dowel in the drill and put your shell over it, it should fit nice and tight. This whole thing will act as a simple lathe, perfect for quick sanding. I was using 400 grit and 800 grit sandpaper to clean up the shell.

Step 3: Cutting to the Size.

Measure 8mm from the open end of the shell. To make a perfectly straight cut, clamp the bullet shell in to the drill and cut it with a hacksaw with the drill rotating clockwise.

After sanding off the sharp edges, the length of the shell should be approximately 12mm.

After cutting you can buff the shell with high grit sandpaper (1000 or more) or use steel wool.

Step 4: Disassembling Old Headphones

Step 5: Assembling New Headphones

It's very important not to forget to put the wire through the hole in the shell before soldering it to the headphone speaker. I did that by mistake. =)

By some mystical coincidence the metal top of my old headphones fit perfectly snug in the .40 cal bullet shell. All I needed to do was to fit them together using the vice. However, if your headphones and bullet shells, don't have a snug fit, you can use superglue.