Well, I was curious, too, so I ordered 50 3/8" birch dowels from American Woodcrafters. They are all straight, 36" long, and look like they would make decent arrows. Some grain runout, and a few knots here and there, but for the most part they appear to be serviceable. Out of the 50, here's how they spined out:
68-1 70-1 73-2 75-3 76-2 79-1 81-1 84-1 87-7 90-2 93-4 96-7 98-1 100-5 104-8 108-3 130-1
Those are WAY heavier than I thought they would be, but I wanted to see for myself.
The 5/16" I have used in the past maxed out at 55#, with the majority going in the low to mid 40's.
Just thought you'd like to know.