looking at the pics of the mag, it appears that the FS rounds are rotated 90* to the left midway down the front of the mag. This would allow them to do a 180* turn at the bottom of the mag and come back up the rear of the mag. 15-20 rds is my guess. Alot like the old RAP4 mags, but for First Strikes. As I recall, RAP4 showed a similar mag design in one of their videos as an additional type of dmag they were planning to produce.