Sorry I didn't respond earlier - I haven't been back on the computer since last night. The loads I listed in my original post on this thread are using 44 Magnum brass. My Speer manuals show 6.3 grains of Unique as being max behind Speer's 240 grain swaged lead SWCs when using 44 Special brass.
I've been using 7.2 grains of Unique in 44 Magnum brass behind Speer's lead SWC's for years in Rugers and Smiths with no leading. Several of the others are right though - if you push that swaged lead bullet too fast, it will lead because it's too soft. But those lead SWCs from Speer are swaged, not cast. Harder, cast bullets are a different story. I've found that my own cast SWCs, which are pretty hard, will occasionally lead if I don't push them fast enough. I've always attributed it to my not having enough pressure to cause the bullets to obturate properly, but it could have been the lube or just the guns. I think what someone else mentioned is true also - some guns seem to just be prone to leading. My 45 Colt Rugers were bad for it until I had their cylinder throats reamed.