Commercial heavy bags are never filled with sand,they are filled with shreded
rags.If you filled a 70lb size heavy bag with,it would weigh a couple of
hundred pouds and be as hard as cement.You could not punch it for a full
workout.
Heavy bags are desined to "give" slightly but provide resistance
to develope power over time.
I have a small sand bag, it is about 2 ft long and 12 inch in diameter,
filled wth sand itweighs 40 or 50 lbs.I use it like a hanging Iron palm bag
First smooth,then fast.
Smooth is fast.