Sometimes the aesthetic of a specific pen is not mirrored with the refill it contains. Case in point, the Fisher Space Pen. I love the simple good looks of the bullet pen but I have no need for a thick ballpoint that can write at zero Gs. I like fine, fine, fine gel pen refills. So, I hacked it.
With a little bit of washi tape around the barrel in key points and a trim to the end of the refill, a standard Uni Style Fit Gel Cartridge fits into the Fisher Space Pen like a champ.
I suspect with some finagling, other refills of the Pilot G2 variety might also work. More tests and experiments to follow.