You have decent accuracy with jacketed bullets but the others are not acceptable . I'm beginning to wonder if you have a " timing " problem . The cylinder is not fully lined up with the barrel when you fire a round. Jacketed bullets would be more forgiving , they won't shave off any of the bullet . If the hand (pawl ) is just a tad too long or the crane is bent , that can cause timing problems . Understand , most revolvers , (smith & wesson included ) allow a bit , a very small bit of free rotation to compensate for this . But maybe your is just too far off to compensate . Without being able to look at the item , it's hard for me to really say what the problem is . I will tell you this though , if / when you find it , you will have a revolver that is as accurate as your 357's . Don't give up , there's an answer you just have to dig deeper . A good gunsmith will find it and correct it . Sending it back to the factory , explaining thoroughly what is the problem could be the best answer if a good gunsmith isn't available there . I would not give up and settle for only jacketed bullets or get disgusted and sell it . There's an answer for what's causing accuracy problems .
As far as your problem with unburnt powder , maybe you are flaring the case mouth too much and giving up a lot of " neck tension " . The crimp does very little to hold the bullet long enough for the powder to completely burn . Neck tension is very important . Just flare the case barely enough to " just " start to accept the base of the bullet and no more . I use fast burning powders on my target loads in 44's , using lighter bullets sometimes . Your powder selection , probably is fine .