i just finished loading for the quigley and have a drop tube question. this year i was volunteered, to load for all my shooting family ~ 700 rounds total most in 45-70. as i dutifully dumped powder and carefully dribbled  it into a 28 inch long drop tube i began to wonder what if any real difference it really made. is this really a thing or just some silly thing that has been perpetuated through time. so i took the 65 grain loading of 2f olde Eynsford that i just dropped down the tube put a card wad on top of it and pushed it onto the top of the powder with an empty 223 case. i did this with two fingers and stopped pushing as soon as the wad contacted the powder. then i dumped the same measure of powder into another case. this was done through a funnel with a 4 inch extension on it, all the powder was dumped into the funnel all at once. a card wad was placed on this and again pushed it into place with the empty case. i then used the stem on my caliper to see how far down the case the powder columns were in the respective cases. I was shocked to see only a .005 difference. WHAT WAIT A SQUIRREL!! what the hell is going on here. i spent the next 50 cases and an extra hour of time checking this with every drop tube technique i had ever read or heard about and never saw more than .012 difference in the height of the powder column in the case.  i tried dumping all at once to trickling the powder into the funnel over 45-50 seconds. whatever i did on the drop tube i tried to do exactly the same without the drop tube, every time pushing the card wad onto the powder until it just touched with the 223 case. and then measuring with the caliper stem. the greatest difference i noted over the course of fifty cases was .027 thousandths  of an inch. 

now this certainly was not a scientific test but does cause me to wonder about the need to use a drop tube.  any one else ever actually checked?   

I shot some of these loads side by side with a garmin crono, ten shots five of each with a 7.3 sd

I drop tube my BPCR rounds, and noticed one day I must have gotten lazy or distracted.  I had a particularly dense lot of powder,  so my load was 83gr (yes, I load it hot, but it works). After I drop tube my rounds, the powder level  in the case was all over the place. Did some checking and determined that there was a significant difference in powder level depending of how fast i dumped the powder. I now dump it to a count of 5.

I have not tested, but I suspect consistency is key. Would be interesting to see if there was a difference in not drop tubing at all, vs drop tubing, but I can tell you that dropping at different speeds can foul things up.

Drop tubing also seems to allow more powder in the case due to the powder settling more evenly, but it may not matter on a more modest load

I can see how compression will change the level of equal charges to make them all the same height. But when I started out making up my first BP loads using just a funnel to fill the cases, the fill height of 40 cases was all over the place in height. I could vibrate those cases on the bases and all would settle to equal height. I could drop tube them also and all were the same height. 
My conclusion was that using a drop tube was the easiest way to get the full charges I wanted and have them all the same height. Without a drop tube I couldn't even get the charges low enough to allow for a wad and bullet after just using a funnel.
And as mentioned above, how fast the powder is dropped also changed fill height.