How does Double Tap do it? Their listed velocities tend to be much greater than any of the commercial manufacturers and they apparently do it with less pressure. Is it snake oil or do they really know something that no one else does?
I know nothing about Double Tap, but some of the tricks to make your ammo have higher velosity it to use a longer barrel, or you can compair revolver velosities to pistol velosities where the chamber is in the barrel. There are definately some powders that are better matched to the round and they will give you more velosity with less presure than others.
I would first make sure the you are comparing same weight of bullets. There are a couple makers of self denfence ammo that use super light bullets and put them faster. I have never used their ammo but I have heard good thing. Jovan