BD certainly doesn't explode before Gai can use AT, not when hes already in 7G. After the BD is charged (AT would already hit its target by then) it shoots and the way they explode has varied in the series but most of the time, it explodes once it gets far off. It can even rip through solid objects and still not explode (Leaf Village, Rashomon Gates, Earth Wall etc.) until it reaches a certain point. I don't know why that is but it just is. Either way AT has better feats of traveling speed and the speed at which it explodes once it hits its target.