There is a middle ground.
You do NOT need a lot of powder to knock down those pesky tin zombies that flop down. In fact, knockdowns should be set to fall with a regular 20 gauge load. I have found that about 60 grains of FFg, whatever you can get at a reasonable price, under 7/8 or 1 ounce of #7 or#8 will do the trick. As cases vary in capacity to some extent, you build a load to suit.
- Size and reprime
- scoop or measure the powder and pour it in. 60 gr., or even less might get the job done
- Here is my trick to prevent plastic streaks in the barrel;- put in a 12 ga. card wad (Punch out a card from a cereal box with a standard 3/4 inch punch if you don't have anything else) Omit this step if you can deal with the "snot" by other means
- select a plastic wad for a 1 1/4 oz. field load (Try different wads; the Red wad might even work)
-drop enough shot to bring the load up to a level that allows a good crimp (An adjustible shot measure helps (from LEE) and measure or weigh this so it can be repeated)
For BP you do not need to be a slave to a reloading table as long as you don't use a greater VOLUME of powder than shot, and can even go with less than this by up to 30%. Generally less powder means a tighter pattern. "Less powder, more lead, make birds (or knockdowns) fall dead!"
One of my wad columns for some loads starts with a card overpowder wad, followed by enough (The scientific cut&try method) fiber wads to build up the desired height and topped with;
EITHER a cup-end from a plastic wad - cup facing up then shot to suit
OR, where a shot protector is required slip one in, cut off a regular wad, then shot.
For more advice there is a good thread in THE DARK ARTS