Hmm, to be honest it's a little of all of that Matt. Yes, muzzle velocity is about 3200 feet per second so the bullet breaks the sound barrier by a long way. Once it leaves the muzzle though the bullet will hit it's target well before the sound gets there. I have had animals look around and stay put and also bolt at the sudden dropping of a buddy nearby. Kangaroo's are unpredictable like that. 

I rarely shoot with them on the run as I need to *head-shoot* everything and at the ranges I shoot at that's not easy if they are on the hop. If I was to miss and clip the animal it may escape before I can finish it which is inhumane. So, assuming there's three targets I will take the furthest one first then if/when the other's bolt either follow up really quickly or wait until they settle, if they settle. I may be 300-400 metres away so it's easy to plan the shots generally.

There's no real science to it. I cull on foot, not from a vehicle so I'm not out there popping 20 or 30 a night. I may get  between 1 to 5. Most was 10 but that was unusual. I generally shoot prone unless I'm moving and see a fox. If I do I rack and shoot from any position as there's no *head-shot* law on foxes and a hit from my 6mm rifle ends it no matter where I hit it.

Hope that helps.