Just wondering. I bought a Pre-ban Rock River upper a few months ago that I thought was a M4 Tactical with a chrome-lined bore. I compared it to another upper that I have that I know is a chrome-lined upper and it looks identical, with a small chrome ring around the chamber and bore when looking directly at them. Anyway, the upper was fired on a Bushmaster Pre-ban upper to zero, cleaned, and then left in a non-airtight storage container in my basement. I took it out to clean and noticed a very light coating of rust in the barrel. It cleaned right out, and now the bore is perfect, but can Chrome-lined bores develop surface rust, or did I get a non-chrome lined barrel?  I know it wasn't stored in perfect conditions, I'm going to fix that today.

Prolonged moisture in the right temps will pitt a barrel.

I am under the opinion that any metal can rust given enough time and the right conditions.

When you put it away, was the barrel warm at all?

Quoted: I am under the opinion that any metal can rust given enough time and the right conditions.

Not all metal - only ferrous metals. Gold cannot rust - rust is a form of oxidation and some form of ferrous material must be present in the metal for rust to occur, althought metals such as aluminum can oxidize and corrode, they don't rust in the way iron does.

Quoted: Had you fired steel jacketed ammo through it? Yes, chrome can rust, especially if the chroming was done poorly. You can check to see if it's chrome by degreasing the "chrome" ring at the chamber and applying some cold blue on a swab. Chrome won't cold blue.

Yeah, I hadn't thought of that. I do remember not getting it my usual white glove clean because of time, so maybe that was it.  The upper was Probably a little warm as well, it was a very hot day here in Bama the last time I took it to the range.  I shot another rifle the same time, cleaned it the same, but it was stored upstairs in a semi climate controlled environement and the bore is still perfect on that one. I guess it was just where I was storing it, just wanted some reassurance. Thanks all.

Quoted: Had you fired steel jacketed ammo through it? Yes, chrome can rust, especially if the chroming was done poorly. You can check to see if it's chrome by degreasing the "chrome" ring at the chamber and applying some cold blue on a swab. Chrome won't cold blue.

what he said