Thanks everyone for the very useful info.
Greywolf, you mention:
I understand ACTS and PROVE it safe when you pick up a firearm, but why do it again when putting it down if it hasn't left your hands (direct control) though I know the action should be left open if possible? Or do you mean for example, the instructor asks you to pick up firearm X, load it with the correct cartridge, unload it, then put it away. Then I understand you have to ACTS/PROVE it safe when picked up, load with correct cartridge as per head and data stamps, unload it and ACTS/PROVE it safe before putting it down.
Thanks.