To C. Sumpkin.
Oh yes, it's the length. my Sierra 55 gr HP (flat bottom) mikes at 0.730" and the Hornady 75 gr BTHP at 0.996"
And a cylindrical powder requires more space in a cartridge than a ball powder. I believe there's a difference in length between the Varget and 4064 powders. (cylindrical powder) Cylindrical powders are also more difficult to maintain a consistent load because of their shape. (+/-) 1 - 2 gr for an automatic powder dispenser , like mine on my progressive press. +/- 1 - 2 grs in a small cartridge, like the 223 , 9mm or 45 can make or break a tight group.
That needs more explanation.
Lets say you hand load different rounds with different grains of powder and you find out that you get a 1" group at 100 yds with 25.0 grs, but when you tried 24.8 gr, the group was 3.0". (It can make that much difference)
So now you go to your press, and keep adjusting the auto powder dispenser until most but never all, are at 25.0 grs. You'll actually find out that some will be 24.8, 24.9, 25.0, 25.1, 25.2, especially with a cylindrical powder. The smaller the powder cylinder, the more consistent the measure will be. So even if your powder measure is a good one, it's still going to be pushing out 24.9 to 5.1 grs on a lot of rounds. (+/- 1 gr) You keep checking the output on every 10 rds and keep the powder measure barrel filled to approximately the same height.
The only way to be more accurate is to hand measure each and every powder load. That may be fine for competition or long range hunting (Antelope & Dall Sheep come to mind), but for pistols and magazine fed rifles, it's totally impractical and overly time-consuming. (barring competition)
Hope this helps.