Military rifles are built Ford Truck Tough and they do not stop wars when its raining.
And in bad weather you simply carry the rifle muzzle down, but any oil or fluid in the chamber or bore can increase bolt thrust and chamber pressure.
If the rifle is repeatedly fired under these conditions the headspace can increase from the excessive bolt thrust.
The British military to this day proof tests their small arms with two oiled proof test cartridges. If the headspace increases .003 or more the rifle fails proof testing.
So the two detrimental effects are increased headspace from bolt thrust and rust. And WD-40 displaces moisture and if you ever had condensation inside your distributor cap and the engine wouldn't start, a shot of WD-40 inside the cap will get the engine running.
Below is from the 1929 British Textbook of Small Arms, the British used the base crusher system. The base copper crusher was located at the base of the case and the case had to be oiled to read total chamber pressure.
