Nitro deposits is carbon residue left over from "nitrocellulose" or smokeless gunpowder burning.
In simple terms, powder & primer fouling.
Or the reason you are cleaning the gun in the first place.
It is a very old term dating from black-powder days.
Then, black powder fouling was cleaned out with boiling hot water, followed by drying the water off of the gun & bore, followed by oiling everything, followed by doing it all over again the next day or two..
Then they invented smokeless, or Nitro powder, and then they invented Nitro Powder solvent to clean it with.
And then you only had to clean it once and you were done.
Without boiling water and burning yourself two days in a row.
rc