"Corrosive" ammo means that the primer uses Potassium Chlorate instead of Lead Azide. When it fires, it burns to Potassium Chloride. The first cousin to Sodium Chloride- table salt. left uncleaned in the barrel, it will start the steel to rusting. Do it enuff times, rust will eat pits in the barrel,
To clean the barrel, remove bolt, use funnel in the chamber, pour a quart of HOT water thru the barrel, then put a couple of drops of Dawn dish detergent on a bore brush, and scrub one minute. Pour 2 more quarts of hot water slowly down the barrel, let drip. Dry patch. Let set for 10 minutes, then run an oiled patch down the barrel. Done. Do it again in a week to get any salt that was hiding from you in the grooves.
Look at the bore now- unless it looks like it has acne, will probably shoot OK. These are NOT target rifles- OK means you can hit a man at 400 meters.
What we have here is... failure- to communicate.