S&W Model 10-5 Lead buildup just ahead of forcing cone?
I had tried out my Model 10-5 in an outdoor range session using some of the only .38 special that I was able to find at retail from around last November: HSM Cowboy. They were using Starline brass and had some pretty nice RNFP boolits in them. I'd assume they comply to some sort of historical accuracy in terms of shape, powder, and lubricant. They had felt like lighter loads and had a very pleasant smelling smoke to them. Considering their price, I wouldn't get them again, but it was what I had in my inventory in terms of factory .38 Special.
I had used around 36 of them at my range session. When I cleaned my 10-5, I had noticed leading in my barrel for the first time I'd had it, right ahead of the forcing cone, at the start of the rifling in the barrel. It's very mild, but it certainly was something to note. The rest of the bore was sparkling clean, which I'd have to admit, made the leading look very obvious. I doubt the amount of leading would be detrimental to accuracy or safety, but it's clearly something to make note of and clean up.
I've had a good experience with using Boolits sized to .357 and .358 so far, but I'd assume the area where the rifling starts is going to be the most likely spot for the leading to occur.