Wrong again Jagermeister. The size difference between American #7 and #7 1/2 shot is more like .18 mm
The problem is due to the fact that different countries producing shot shells often use different shot sizes assigning same or similar arbitrary shot numbers.
Shot size does make a big difference. I remember when I picked up my first shotgun and few boxes of shells some Winchester XX and Remington Peters. Not knowing anything about shot sizes I learned that red shells with XX were more effective while yellow ones weren't as good. Later I learned that Winchester were buffered #4 and Remingtons were 7.5.
