Actually 5/16" in decimals is .3125 A perfect fit would be a 2212 with an inside diameter of .3197 The biggest problem, I've found, working with bamboo is that they will not be a consistent 5/16", so I've wound up with several aluminum shaft sizes, and using the one that comes closest to the bamboo shaft I'm currently working on. So far I've used, 2212,2213,2216,2219. All 11/32 O.D. with different I.D.s I've expeirmented using the H.I.T. inserts, and that works well, if you drill the bamboo correctly. (put the footings on before drilling, or you run the risk of splitting the boo).I use jigs I've made-up.
I intend to try the extra long footings with regular Easton inserts, but have not yet.