6-6 is read (Diameter - "Effective" Grip Length) Effective grip length is less than the length of the rivet tube.

6-6 rivets are 3/16" diameter x 3/8" grip length.

Diameter is by 1/32" increments (6/32" = 3/16")

Grip length is by 1/16" increments (6/16" = 3/8")

Looks like the rivets are 1/8" longer than the "Grip Length". The pin has to pull into the rivet tube a bit to expand it enough to work. I measured a 6-10 rivet tube length as 6/8" (grip length is quoted as 5/8" ).