Mark — good question. FOC stands for Forward of Center balance. Somehow the B got deleted. It’s the point that an arrow balances along the shaft ahead of the linear center point of that shaft, expressed as a percentage. EFOC is Extreme foc, generally considered 20% and above. You may also encounter UFOC, or Ultra, 25% and higher and darn hard to attain. For a simple way to calculate FOC see the conversion chart in the Ashby Library on this site. The buzz about FOC is the recent revelation from the ongoing Ashby Study that it’s among the most important aspects of gaining good lethal arrow penetration (along with overall arrow weight and broadhead design). Another bonus is that optimized FOC provides a higher percentage gain in penetration with lighter bows than with heavier bows, which is good news to younger and older shooters and petite women. Scan through the Ashby Library, looking at the more recent study updates and you’ll find a wealth of great info. Ashby has also contributed several articles on the topic to TBM with more on the way. Hope that helps, dave
