The adhesion of a foreign material onto a surface, especially so as to reduce its functionality(noun)
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Use "fouling" in a sentence
"= The fouling which results from firing is of two kinds -- the _powder fouling_, from the burning of the powder; and the _metal fouling_, from the nickel scraped off the bullet as it passes through the bore."
"Sweets 7.62 when copper fouling is detected followed by Hoppes No. 9 0"
"That makes it ideal for a soak to remove copper fouling from a rifle's bore."