I EDC a Zebra 701, its not small like the compact 301, but its built like a tank. I bet an industrious guy could cut one down to take the compact refills .... that would be cool.
I like my Pilot G-2 Mini ...but its 100% plastic..so easily broken, even though mine isn't, it still concerns me. Not an issue with the F-701.
As for the Space Pen crowd ...I too have several pens that take Space Pen cartridges and have bought new refills thinking I got a bad one every time it globbed ink onto the paper until I finally gave up. I over it. Zebra pens work fin ...they are not very pricey and I can run the cartridges dry satisfying my desire to use it up before trashing it.