Bohning's F Nock fits micro shafts with an ID of .166" (commonly known as 4mm or G Nock) shafts and G UNI Bushing systems. The F Nock features a double-lock throat of .120" (3.05mm). Each F Nock weighs an average of 6.5 grains (421.2 mg). Tech tip: for easier insertion and indexing, apply a small amount of Bohning's Tex-Tite or Seal-Tite Wax to the nock shank. At Bohning, every nock style must pass the following steps before introduction to the market: 1. Computer simulations identify potential weak points. 2. Destructive testing methods expose possible design flaws or material defects. 3. Accelerated life cycle testing using a custom-built bow is performed at kinetic energy levels exceeding 110 ft-lbs (nearly twice that of most hunting bows). A single nock must withstand over 1000 shots or it goes back to step 1.