Vp9 questions

So this is my first firearm ever! Only used about 300 rounds so far and I do clean as thorough as I can after each outing. Few days ago after some inspections I noticed this brown discoloration around the welding points. Not sure if it's "normal" wear from firing, or normal for HK to have these brown coloring. Please let me know what your opinions are on this subject. Would like to know if it's a safety hazard or durability concern for later down the road. Thanks!