This is going to be a very elementary question.....What is the difference (other than weight) between the 300, 340 and 400 carbon arrows? I originally thought it was the ID of the shaft but when shopping for inserts, there is only one size. Is it the OD?Or just the density? My buddy and I are going round and round via email with various arrow weight calculations and its killing us......He's got 340's and I've got 400's but neither of us have both to compare until we meet to go hunting.....

It's a spine rating. Spine deflection.

And what THAT means the sitffness of the arrow. Basically .300, .340, .400 etc is determined by hanging a weight from the shaft supported at both ends, then they measure the deflection (or the distance the arrow sags from horizontal. So the lower the number, the stiffer the shaft. They're heavier because they need more material to make them stronger.