Hi,
I still have a big bag of 5/16 UNF x 1.1/4" with hexagon socket caps.
I use(d) these for attaching various weird and wonderful weights to my Goldmedallist.
I had some success in bonding the top part of the bolt into a small plastic cylinder, and using electronics encapsulating resin to fill up the interior (this resin is heavy in itself, but I buried some large washers in there as well for extra mass).
The resin gives some shock absorption, without actually using 'shock absorption devices' to attach the weight, and the cylinder is tall enough to move the weight out and away from the centreline of the bow, without actually using a 'rod' - it's a one-peice weight.
AIUK reader 'Bundy' is still using one of these on her barebow Winstar, officials are yet to complain!
I have also used these bolts to attach a small (but weighty) stainless steel pressure manifold to the bottom of the riser, which completed the balance of the bare goldmedallist.
The next stage was to be making weights by casting the bolt heads with smelted lead in a sand-mould, and then coating with something called 'plasti-dip' to make it safe and pretty. But I've bought the Nilo now, so my experiements have come to a halt.
If you feel the need to experiment yourself, I can send you some bolts in the post.