If your lead is good then it has to be the powder. I've always used fine in my P.61 but with medium I would put the load up to at least 80 grs. particularly with low cost powders like Henry Kranks generic. Swiss and Vectan powders are another matter. If bullets fly straight at this load then decrease the load until you reach the point they tumble. The fact is you need enough breech pressure to upset the bullet at the outset. Failing to do this will let the propellent gasses to blow by, effectively floating to bullet off the barrel wall. The change in performance in the two conditions is quite marked, for a start you get a much more authoritative sound from the muzzle blast. You do not get a condition of slightly obturating or mostly obturating; it either is or it isn't.

I witnessed someone shooting a 2 Band at 100 yards who thought he could reduce the powder charge to just 60gr. to reduce the cost of shooting. Every bullet hit the range floor about ten yards from the target and I could actually hear the warbling sound of the bullets flying end over end. He did manage a couple on the target but only because the target was in way as the bullet bounced up in to the backstop It took me a lot of persuading to to convince him to up the powder charge. Just an extra 5 gr. made the difference; snappier muzzle report and round holes in the black. The extra five gr. would have only added another 60fps at most so that in itself couldn't account for the 3 feet+ change in the height of impact. What made the difference was that the powder gases now exited after the bullet instead of before.

Do not bother with drop tubes for your powder you are just complicating things. At this stage you need to concentrate on getting round holes in your target. With both my P.61 carbine and P.53 "3 band" rifle my loading process is powder into measure then into barrel followed by the bullet rammed home with the ramrod, cap on, fire. No fouling shots, no swabbing out, no golf caddy full of rods and brushes. BTW, you do know to always shoot with the ram rod in place don't you? The spring that grips the ram rod is tensioned against the barrel, failure to have the ram rod in place will (usually) cause bullets to go any where other than where you point the rifle.

One more thing, the slide top 100 round boxes that Remington .22 LR comes in are just the right size to take 20 .577 Minies.