10gauage, you're doing everything the right way in trying all the different powder styles and bullet choices. It takes time. The whole process might be easier though if you separate your comparing sidelock from the in-line.
First: understand that in-lines typically burn hotter than the sidelocks and in doing so create stuff that cooks on much harder than anything a sidelock can do.
Second: there's a whole world of difference between the different powders that can be used in in-lines [you already know that], but understand that as the powders you use get more advanced [T7 & BH as opposed to, say, pyrodex and true black] the more the after-shot issues become complex, like the crud ring and cleaning or clearing it. Flash channel and flame hole issues mount with some of the more advanced powders too, as you're finding out. Drop-down action, break or bolt, if you're using the more complex powder types you're going to have to work thru these issues as they pop up in a way that you can live with at the range or in the field. You see to be doing that and doing it well.
Dealing with the issues you are can be frustrating, but you'll prevail. When I made the transition from sidelock to in-line I started with a bolt gun because I was very familiar with my center-fires which were all bolts, thinking the new muzzy wouldn't be any different. I shot pellets in it and got a rude awakening, so I tried a half dozen different powders including two or three blacks but felt I was going backwards. I finally discovered the BH209 and thought I'd gone to heaven, yet I still had issues that I needed to overcome. Today I shoot a blended powder selection in that the range time sees T7 granular and the deer hunting sees the BH209. Each powder has its own challenges, and you'll find you way with them. If I were to offer anything at this point it would be to get away from the use of pellets simply so you don't end up settling on a charge that may be a limiting factor for something better.
I've been following this thread because I can see so much of the path I was on in it.... lots of parallels here. Enjoy the journey because it will have some super rewards in the end.
