You also need to consider the rifle crack - get a good moddy on it and your neighbours may not even realise that you are shooting. If you are thinking of a new rifle the HW100 Mk2 is very quite for an off the shelf rifle
Dry sand is excellent and self healing if kept dry. Depends if you want portability or not. Layers of carpet also good and rotate them. Have a word with your local carpet shop to see if they have any mis shapen off cuts. If you are for squares or specific dimensions, they will possibly try to charge you.
The lead is also very silent as the round melts into the mass. But be careful with molten metals and the fumes anr noxious so well ventilated and stay upwind.
In a battle of wits I refuse to engage with an unarmed person.
To one shot one kill, you need to seek the S. Kill only comes from Skill