yep, like john said - the jig will work just fine with 5/16" shafts by wrapping with tape for a snug fit inside the jig, which insures the shaft end will be centered properly. you can reuse the tape a whole buncha times, too.
after the initial hole is drilled, i remove the jig and run the drill bit in as far as it will go, to allow for more nail. if i don't wanna add weight, just stiffness, i use a bamboo bbq skewer instead of a nail.
also, i like using iron nails for the foot weight cause they're cheap and heavy enuf. just dremel or hacksaw off the head, resulting weight is shy of 70 grains, trim and weigh on a digital scale to match up all the weights. epoxy or cya glue'em in.