This makes more sense. Without a pressure build up there is not a lot of velocity. Once the rifling has been engraved, it takes very little pressure to continue to move the bullet down the barrel. This is why a large percentage of your velocity is gained in the first few inches of barrel. That is where the pressure builds as the bullet is engraved, then drops drastically shortly afterward. Even the slowest powder has all been consumed by the time the bullet is maybe 12 inches down the barrel, so no more pressure is being gained. with drive bands, if the pressure never gets that high, then a faster powder is needed to gain the velocity.