Okay. Help is here. First off, tell me what kind of ML this is. Make and model. Also tell me the caliber. 50? 54? 45? Is this a brand new one, or is it just new to you?
Second, stop brushing the bore between shots. Over time, that will erode the riflling. It is not the bristles doing the damage but the crud on the bristles. Use a spit patch and then a dry one between shots. That should be enough.
Next, any bullet that falls out the barrel is too small. Those are not for you. Sabots? They're great when they work, but it can be a bear to find the right one. Obviously you have not. I don't know quite what to tell you beyond that for now. Get back with some answers and we will go from there.