Ugh. Just sold two other backs with my 500c body, so this is my only back at the moment.
Sitting here dry firing and it seems to lock up at approximately the same frame each time. What's with that? Does the back circular gear cycle through at this rate? Does it make 2 cycles per roll? If so, that might explain it locking up at the halfway point.
I checked the gears but don't know what to look for. Hmmm..