I don't use a filler. I have done that before and it is alot of trouble for the results you get.
Tip the bore up, before fireing and get the powder stacked up in the case, nest to the primer. It will be accurate enough.
Try 10 gr of unique with a 158gr cast plain base bullet. That will get you about 1300 fps. You can work up, carefully, to 12 gr for 1600 fps with the same bullet.
Slower burning powders, like 2400 or either 4227 will use more powder and get higher velocities. The volume in the case may not be any greater than the Unique though. You probably don't want to shoot a plain base bullet much faster than 1600 fps anyway.
I use 12 gr of Unique with a cast plain base 150 gr bullet in my Model 94 30-30 and get right at 1650 fps. It is accurate and I get no leading.
Sixgun