stone roller, heavy stone, weight, plummet
砣 centers on a heavy stone object used for pressing, rolling, or weighing things down: a stone roller, a weight, or a plummet. The idea is a dense stone mass that applies force or measures weight.
砣 combines 石 (stone) with 它, which likely contributes the sound. The character is used for stone weights and rollers, fitting the stone semantic.
A stone (石) on the left and a shape like a heavy load (它) on the right: imagine a massive stone roller pressing down, used as a weight or plummet.
For タ, picture a heavy stone roller pressing down on a tar surface: tar -> タ, and the stone weight flattens it.