Corbanzo, has just about hit it. The real trick to mechanical powder measures is the consistency of your operation of the measure. That means every phase of its operation, from putting a baffle in the bottom of the hopper, how firmly you operate the handle, how smoothly or roughly you operate it, "bumping" the stops the same way each stroke, dumping back a charge that didn't "feel" just right, keeping the powder level consistent. It will be obvious that some powders meter more easily that others, but ALL will benefit from your contributions. Even long stick powders can be thrown with surprising results. Once my measure is set, I do not weigh individual charges, whether pistol or rifle other than periodic testing. Neither the factories nor the vast majority of target shooters weigh their charges, why should I? Remember, the volume of the measure doesn't change from stroke to stroke, only your delivery of the stroke itself. Any of the rotary chamber measures from Hornady, RCBS or Redding (even Lee) will serve for 98% of the duties that a measure could be called upon.
You will be amazed how accurate a measure can be once you correct and organize your own procedures.