First, lead SWC ammunition today is not at all common, in fact I don't believe I've seen any lead SWC commercial ammunition in at least 15 years. This leads me to believe that you are building your own ammunition. As a reloader I have no problem at all with your doing this but you may be blaming some factors created by your choice of powder and method of priming your cases on your model 66.

Btw I had about 500 rounds of 38 special with poorly seated primers due to my using a hand primer that was basically worn out. So I have first hand experience with what happens with poorly seated primers. Good new is they will go off if you hit them a second time but by golly you will get a real education on whether you are flinching while working through 500 rounds of ammo with poorly seated primers.

Some powders also shoot rather dirty. My experience with CFE223 in my AR's is that it will leave the receiver saturated with a gummy black mess after only 50 or 60 rounds while load built using Varget shoot much cleaner. I suspect the difference is due to whatever chemicals are used to reduce copper fouling so CFE pistol is probably similar in terms of fouling. BTW, that copper fouling eliminator is actually more than ad hype, it actually does reduce copper fouling in a barrel. Point being is that Powder Choice could be a factor in the fouling related issues you are dealing with.

Finally I've been pelted often enough at shorter indoor ranges that I won't shoot 357 Magnums at a range shorter than 75 feet. I had one day when I felt I might have problems with my model 620 until I felt a chink of debris hit me in the knee. Since the lanes at this range feature a bench a bit below chest high the ONLY path for something to hit my knee was from the backstop of the 50 foot range I was shooting at.

Finally, you have a warranty on your new pistol. I would suggest that to take advantage of that and call S&W Customer Service.