I usually use steel shot and if I get it wrong, one fix is to drill a hole a little bigger than the shot in an inconspicuous place, hold the model so it's on its nose, dip the shot in something like two part epoxy, CA or carpenters glue, and drop it in. Keep it on its nose until the glue has had time to set so the shot stays in place. The amount of shot can be determined ahead of time by taping a small lightweight container on the nose about where the shot would wind up and putting shot in it until the model just balances, and then put in a little more.