DiGI,
That's right about grains being a weight equivalency. Just to let you know though - If you go to your local hardware store and get dowels, be it poplar, or oak or whatever and are going to start fiddling around making arrows, MAKE SURE that the wood grain isn't running off the shaft, that is, it's straight through the length.
Runoff's on the arrow shaft can split and seriously damage your hand. It's happened!
When it comes to shooting the bow is 50% and the arrows are the other half.