3/56, 6/48, & 8/40 are the most common shotgun sight thread sizes. The first number is the screw size. The second is the threads per inch, i.e., #6 screw with 48 threads per inch. The tap drill for 6/48 is a number 31 drill bit, for a 3/56---a number 45 bit, and for an 8/40---a number 28 bit.
