What is the core diameter on the 600 series? i am hopeing these would be big enough for the axil gs2 extremes they have a diameter of 5.2mm

The inside diameter measures 3/16".

will they work on xfyro earbuds?

Our engineers have not tested those earphones for compatibility with our core-specific tips. Therefore, we highly recommend our SmartCore™ tips. The SmartCore™ tips feature a patented inner core design in order to fit around most earphones. These tips come in our Sport Pro and Audio Pro series so you will get the same features and benefits of our core-specific tips. As always, we want you to be satisfied with the tips you received. If for some reason the tips do not fit your earphones, please let us know and we will make it right! Here is the link: http://www.complyfoam.com/SmartCore/