Sometimes it matters with the ammo you use. Here's one from the logs of J7 experiences. 16 years old, I saved up some money to get a deer rifle, found a used Rem 788 in .308 with a piece of crap 4x, nitrogen filled, K-mart on it. At 16, I was pretty proud of it and liked to shoot it. Just before I went into the Army I found some cheap Portuguese surplus ammo and went out for a shoot. Returned home cleaned it really good and put it in the gun locker.

While I was gone I asked my best friend to come over and run some oil on my toys every once in a while (its real humid in MI during the summer). Came home on leave and just had to inspect my babies. Looked down the barrel of the .308 and there were rust pits. So, I asked my friend WTF didn't you come over while I was gone? He said he did and Mom confirmed. It only happened in this particular rifle. So from then on I was kind of miffed why this only happened in this particular rifle. I eventually traded it off for another toy.

Several years ago I was researching a bulk purchase of 7.62 x 39 and got to reading about corrosive priming. Then, click, OH, sigh. Turns out that surplus ammo was corrosively primed and there's not enough oil in the world that would have prevented it from rusting. I read that I needed to put water, or some guys use windex, down the barrel to get the salts to dissolve and off the metal. At 16, I didn't know anything about corrosive priming, and in my eyes the barrel was ruined. Wish I hadn't of traded it off as it would have been a good candidate for a re-barrel.