no_agenda, if the fouling is near the chamber end and especially hard to clean, you are probably getting gas cutting. Too much charge typically causes leading at the end of the barrel.
Gas cutting occurs because powder charge is too light to obturate the bullet base or bullet hardness is too hard and diameter is too small for the barrel so hot gas is escaping around the bullet. Mind asking which pistol you are shooting out of?
You could cover all bases and test load some at lower/higher charges (say 3.8 and 4.2 gr) and see if the fouling gets better or worse. If the fouling decreases with the higher charge, then lack of obturation is the cause for the fouling.
Four Knives, I have not shot the 115 gr MBC bullet. I do like the 125 gr Small Ball for the LW conversion barrel for G22/G27. I have gotten minimal leading with 4.3 gr of W231/HP38, but I was range testing different OALs from 1.08" to 1.12" so I am not sure which OAL may have caused the leading.
I am planning to do more test loads at 3.8gr /4.0gr using 1.10" OAL. If leading problem gets worse, then I will go back to 4.3gr at 1.10" OAL and see what happens.