Hi Tom,
you are right on the money about the varying POI's. I have found (and so have my buddies) that some days, especially warm, humid days... we can"t beg, borrow, or steal a tight group. I only shoot Pyrodex, and for consistant shooting it has to be fresh, naturally, because as you correctly implied, the powder is hygroscopic. For consistant seating, I usually use a Pack-jack which controls the downward pressure (sort of like a torque wrench for downward pressure). It simply slides over the end of your range rod.
Now let me ask you a question....which would be affected more by a given amount of humidity, a small amount of powder getting slightly damp, or a large amount of powder getting slightly damp? Obviously the small amount would be affected to a greater extent, having a greater effect on consistancy. And believe me, even the larger charge is quite affected. I don't think any ML can always be a tack driver all the time. Bummer!