I don't see how a longer or shorter point can effect spine. If it was a different weight yes, but the length of the broad head or practice tip should not effect spine. I would think the spine is determined by how hard or easy the shaft bends when force is applied to it. Since the point is a solid piece of metal it does not flex, and should have no effect on the spine of the shaft. That would also lead me to believe that the type of insert and length would also effect the spine of an arrow. If you used an insert that was 2 inches long instead of 1 inch, you would have to calculate your shaft as if it were an inch shorter since that part of the shaft would now be solid.

I am not saying that's the way it is, just the way I understand it. Otherwise it would mean when you buy arrows you would have to already know what broad heads you were going to use to get the correct spine. And field tips would never tune properly or fly right. But the exact oppisite is true, field tips fly better.

I think it has more to do with the fact that the broad heads have blades on them that are like having fletchings on the front of your arrows. They are very sensitive to launch geomitry and wind planing. While field points are very arrow dynamic and quite forgiving to small errors. If your arrows were not spined correctly you would not get them to tune well with broad heads and you would not be able to tell what moving your rest a slight amount did to them because they would be all over the place.

Paul