These are not 3/8" cutting diameter. The 3/8" means you can thread mill an internal thread as small as 3/8"-16. Please note;

Specs: 5/16" shank, 0.300" cutting diameter, 4 Flutes, 0.218" neck diameter, 1-1/2" reach depth, 3" overall length. Use for milling 3/8" diameter threads or larger. Internal or external threads from 14-40 TPI.