I get my worst leading with undersized bullet. Perhaps the bullets need to be .001" or more larger. I've also found a leaded barrel leads up even more quickly. I would get some lead remover solvent and go at it till I got a clean patch. Once clean, I would then do a JB-Kroil treatment. I've found this helps with future cleanings. My last step is to put a Kroil wetted patch followed by 2 dry ones down the bore. I believe this help prevent lead build up.

I have a 1894 in 38spl/357mag and it does not like cast bullets at all. It tends to get really bad lead build up after three or four rounds of 38spl, regardless of factory or handloads. That said, do you do anything different when cleaning the barrel? I can never seem to get a clean patch from it with the microgroove barrel.

I treat the barrel initially the same as any rifle. Push it with shooters choice every 3-5 shots. When I get home I let it soak good with shooters choice. Clean it with a nylon brush and Nylon coated rod. Then I scrub it good with JB's bore paste and re-do it all again with shooters choice. I have not had any trouble with lead build up even after pounding a few boxes through it at the range.
I tried the Ultramax 240 grainers yesterday. I wouldn't use those unless you plan on shooting 25 yards or less. They have so little powder its like a cap gun. They were hitting the dirt before they flew 100 yards. I guess mine just took a few hundred rounds to break in. After another box of the 305 grain Buffalo bores my groups started to tighten up to about 1.75-2" three shot groups at 100 yards. Plenty good enough. I do wish I had the other rifeling and a different twist for heavier bullets.