Initially, there were going to be 50 caliber Freak sets. Can't seem to find them anymore, so if I had to guess, I'd be thinking that when KEE picked up the Smart Parts properties, the 50 Freak project died.
We were screwing around a bit with my 50-cal Stormer at the field this weekend. Really, it shot well, as it always has. Had a 10-year old shoot it (not a regular player) at some targets, and she was a better shot than any of the rest of us.
For what it's worth, picking up a 50-cal marker isn't going to be a gigantic expense. They're out there fairly cheap. But, be aware that the paint itself isn't that much cheaper than 68-cal.