The 6 1/2 has a softer cup and a lower brisance. It is very useful in small capcity cases of lower pressure like the 22H, 25-20, 32-20, etc. As mentioned it is not for use in the smaller higher pressure cartridges of the .222+ family. The 7 1/2 primer was developed especially for these smaller higher pressure cartridges. It has a stronger cup and more brisance conducive to the use of ball powders. The WSR primer is pretty much the same as is the Federal 205.

There are small rifle magnum primers such as CCI 450s.

Larry Gibson