Holland and Holland, gunmakers of London, England since 1835, wrote a friend of mine in Smithers concerning his 6 bore SxS ball and shot gun of their make. They told him
to use cold tap water, then dry the bore, then flush the bore with WD40 or other water displacing lubricant, then patch the excess of that out. They then noted, the gun may
or may not need any further rust proofing, depending on location.
Since I read that letter to Wil, back in 1976, I have cleaned my guns exactly like that.
The fact that WD 40 is a petroleum product had no basis in post cleaning application to me. The fouling is gone, I wanted it for the water displacing feature, just as H&H noted.
In our location, no further rust proofing has been necessary since those 1970's. No rust.