Fisher is world renown for its pressurized ballpoint refill which went to the moon. The astronauts used it for writing right side-up, upside down, under water, at zero gravity, at high altitudes, basically - EVERYWHERE. It fits in the Bullet Space Pen, but can also be adapted for use in any ballpoint that uses a Parker-style refill. Colorado Pen carries a large selection of colors and point sizes in this oil-based, waxy ballpoint ink refill.
Colorado Pen also carries a mini ballpoint refill which snaps off at designated lengths so you can adapt it for a proper fit in your particular multi-pen.