I have turned an m16 barrel white, and it didn't take long.
What always strikes me about these incidents is there is always a lot of video where officers are milling around in full tactical gear, pockets and pouches stuffed to the max. As an army veteran I understand that putting on all that garb takes some time. Why didn't they just grab a rifle and show up as soon as possible? I understand that patrolmen probably did that and they called in the tactical squad, but still, not every event needs that gear. I never have liked the police dressed like soldier bit, do I may be biased.