Brian is right about tumbling with stainless steel pins. A few months ago I picked up some 40S&W brass left by a guy running a H&K full auto model something not available to the general public. Asked him about the lines - fluted chamber.
After tumbling with pins, I couldn't tell which pieces of brass were from the H&K.
I've been polishing with stainless pins for a few months. Little more work but well worth it for the results.
All the Best,