Fouling at base of brass??

[mattwashington]

I'm shooting a Tikka 300 Winchester Magnum with Norma brass. 168 gr Sierra HPBT, my powder charge is on the low end. I noticed on 3 of the 25 rounds that I fired have fouling around the base of the brass. I'm also shooting 180 gr Hornady SST with my powder charge on the high end, with no problems.

Does this mean I have a few bad brass? I wouldn't think that with a light charge it would be an overpressure issue.
Thanks for your help.