All pellets from the lightest .177 to the heaviest .25 drop at the same rate... It's called gravity.

However the energy imparted on the pellet by the gun does mean heavier pellets travel slower for the same energy.

But that's not the only factor in the final muzzle velocity, pellet fit consistancy within the barrel contributes too. As well as rifle consistancy.

Aerodynamics of the pellet then come into play once the pellet has left the barrel futher decaying the velocity in flight.

The only reason a pellet drops more at the target is because it's time of flight is longer as it's travelling slower. So has more time to drop.

So it's not just so simple as weight variations.

Tony