Don't have my loading manual in front of me but soot can be a sign of too low of pressure. The pressure is not high enough to expand the brass to the point that it seals against the firing chamber. This all assumes the firing chamber is within spec, too. If your loads are toward the low end, try increasing the charge.

Edit: I just checked my Hornady manual and it shows 4.1 - 4.7 grains for the 115 grain bullet. I know plated bullets should not be loaded hot but I would not go below minimum.