[kje54]

Are the flakes imbeded in the target o just sticking to it? What about the environment? Are you indoors? How about the material lining your booth? You say the flakes are laying on case, how about the celing? Do you use the case as a support while shooting? Could the backstop material be getting erupted into the air by the bullets and floating around?

Like you stated I tend to think it's environmental and not the actual firearm itself.