The problem is not so much the pellet weight difference but what other changes in the pellet the weight difference may indicate. Pellet weight itself has very little effect in the position of the impact point even at 50 yards, though in certain circumstances the difference may be enough to be significant.
Some years ago I modelled the effect of a change in pellet weight, all other factors being kept constant for both a springer and a PCP. The effect for a PCP is much less than it is for the springer due to the basic properties of the different types of rifle. At 50 yards range a change from 8.3 grains to 8.5 grains produced a change in impact point of less than 0.1 inches. The size of the change is linear with the change in weight. This took into account the change in muzzle velocity as well as the change in flight. For a springer the equivalent figure was around twice as big due to the changes in muzzle energy with weight change. These figures are very small but remember they represent the pure weight change effect only. Other small changes which may go with the weight change (such as start pressure, head size, centre of gravity position etc.) can produce larger figures. The variations in weight will not affect accuracy but they may effect group size.
It is because of the relative size of the effects of other changes in pellet characteristics compared to the weight effect that it is not possible to experimentally see the pure weight effect as no two pellets, even of the same weight, are ever identical.
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Originally Posted by bozzer
