6, .7, and .8 grams of Dupont rifle powder
What are you actually loading? A gram is a unit of weight equivalent to 27 grains. You are not loading 16, 18.9, and 21.6 grains of powder into that little case.
What are you using to weigh and to dispense the powder?
think a slight thinning of the base just enough to remove .44 Special
You might try thinning the rim from the other side. The problem with thinning from the head stamp side is that you also change the depth of the primer pocket.
It might also be helpful if you would stop thinking in terms of "pistol" black powder.....there is no "pistol" black. There are two granulations commonly used for both rifles and pistols, FFFg and FFg. FFg is frequently used in BP shotgun loads also. Whatever the DuPont powder you are using is, I am not familiar with it.
Did you slug the bore yet?
The proper bullet for the .44 Bulldog and it's longer brother, the .44 Webley, is a heeled bullet (.22 Rimfire bullets are heeled).