The .375 Caliber Bullet Space Pen uses a .375 H&H Magnum shell as a cap combined with a Fisher space pen. The pen is designed to fit into the case, writing side out, for a “…balanced and sturdy writing instrument.”
For those unfamiliar with the concept of a space pen, it utilizes thixotropic ink and a pressurized chamber that allows the pen to be used upside down, underwater, on oil and grease, and in temperature ranges from -30F/-35C to 250F/121C.