most magazines don't use all the available space at all from what I've seen. the spring compresses down, and in single stacks, it is like 30-40% of the available space is just for the magazine spring to compress. I was looking at some Ruger Standard mags and noticing how the spring sort of sits off to the side and not directly centered below the follower. If someone was to design a magazine that used a spring stretching instead of a spring compressing, in theory anyways, you could quickly turn 8 round single stack magazines into 12 rounds - or possibly 15 round double stack mags into 20 or 22 round magazines. someone would just need to figure out a new mechanism so the follower can travel all the way to the bottom of the magazine. where is John Browning when you need him … lol so say you have an 8 round single stack magazine, and expand that to 12 by modifying the way the spring and follower work, then turn that into a double stack magazine and pretty quickly you have an 18 round magazine, just by some design changes. It probably would not be that hard to engineer, but would be too expensive to be of any real value - nobody would buy it if it cost twice or three times as much.
