My barrel is a 1in12. When the US pushed for NATO acceptance of the 5.56 the Europeans agreed but wanted the Belgium 62gr lead with front hardened steel insert bullet for helmet perforation at 600m over the original 55gr FMJ. Hence the faster twist rate to 1in7. At least that is what I remember.Years ago I dissected one on a mill and the hard insert broke the endmill. The real deal front steel cores are friggin hard.