Flungonin, the arrows were orgininally smaller than 3/8, but I narrowed them to 5/16 and than used another stick and compressed all the fibers down to increase the spine. It worked! The arrows fly tru and straight. I was worried about them wobbling but they haven't. I believe this may have been my forefathers did this. Of course, they barrel tapered theirs as well. I might try that next time. BTW the arrows are 27 knock to tip. fletching is 7 inches with turkey feathers