Pull up a log, and have a seat. Corrosive has to do with primers.
Originally, primers were a bit of fulminate of mercury. Known as mercuric primers, the mercury tended to react with the brass over a period of time when sitting, and at once when shot. The amalgam formed would cause the brass to become brittle. This was replaced with a primer based on potassium chlorate.
Known as corrosive primers, chlorate (KClO3) burns to potassium chloride. Very similar to it's first cousin- sodium chloride (aka table salt). When fired it leaves a trace of chloride in the weapon. If you fail to clean that out, well, salt+ steel+ moisture= rust.
Much of the world began replacing the chlorate primers in the mid 1950s with non-corrosive primers. These were based on lead azide or lead styphanate. Due to health concerns about airborne lead, they are now being replaced by a non lead primer, diazo-dinitro-phenol. Commonly known as DDNP or "dinol."
Many of the communist nations did not switch over to non-corrosive primers- chlorate primers store well over long periods, and do not require stockpiles of strategic materials to make. But (Horrors
) they do make steel rust if you do not clean it properly.
Properly means using something that will dissolve salt. Many cleaning solvents are petroleum based, and do not touch salt. Hmm- what could dissolve salt? Hey! How about WATER ?!?
And THAT is the answer. After shooting corrosive primed ammo, clean the weapon (same day) My technique works for me- Remove bolt, place gun in padded vise, barrel tipped down at a 45 degree angle, bucket sitting on floor under muzzle. Use cheap plastic funnel, pour a quart of HOT water slowly thru chamber. Ewww! LOOK at what came out! YUCK! Then take rod with brush, put a couple of drops of Dawn dish detergent on brush, scrub bore. Run another TWO quarts of HOT water thru barrel, let drain. While that is draining (and drying from the heat) take gas tube and piston, wash under hot water in sink. Let that drain, wipe down bolt with damp cloth. After things dry, oily patch thru the bore, light lube on other parts- you are done. 5 minutes + drying time (10 minutes) Some folks use Windex, but it is mainly the water in Windex that does the work.