Hope this helps(that was funny Giz). It will start out as a light 'freckling' and if not taken care of it will turn to rust spots that are through the finish. Here is some on an almost 100 yr old gun. James
Yeah, I know what pitting is, as many old ratty guns as I've had in my life.
Pitting is sad. It is a telltale sign that someone had no pride and was careless with his gun. Except perhaps in an extended military campaign it shouldn't occur. One can drop his favorite shotgun in the lake while duck hunting and still not have pitting if he takes responsibility to properly care for the sopping gun afterward.
It can happen very fast in salty humid climates like florida.Especialy if a gun is stored liked in the presentation case and left for over a month with out cleaning and oiling.The side it is laying on can pit very fast.
In the south east where swamp coolers are used and you get some humid days during monsoon season the swamp coolers will cause even more humidity and rust any thing in your house and warp any thing made of wood.It can start in days under conditions like that.
You can also get pitting inside your barrel in the riflings.Best store a gun in a good safe or a closet where the air is dry.When you clean and oil a gun you want to avoid touching any part of the steel when you store it. Because your hands have body oil and salt in them and the pitting if it happens will start where your finger prints are left on the steel.
It can all be avoided buy just regular cleaning and in humid periods make sure to clean and oil more often.If you ever buy a nickle plated gun be careful not to get bore cleaner on the nickle finish. If there is a microscopic spot on the finish and the bore cleaner gets on it. It will penetrate down under the nickle to the copper and eat the copper away and start pitting under the nickle and start to bubble and flake off.
I have a hunting rifle that is over forty years old that had been soaking wet more times that I can count from hunting in rain and snow.There is a not a pit any where on it.You have to make sure to clean your gun after every time you use it before you put it away.The out side and the bore and mechanisms.