I had an issue with 18-1/2" barrels -- two of them -- on the 590-A1's basically when they first came out. I own many Mossbergs and factory accessories and never had any other problems. In these two barrels there was melted steel inside the muzzles where the front sight were put in. I say steel because I could not scratch it and they had to replace both.
No problems before or since. Great customer service.
Oh, one other issue... Sent in an 18-1/2" 500 barrel to be ported. They reblued it instead (I DID complain about the bluing in my letter in their defense). I sent it back and they simply shipped me a brand new ported barrel -- lol.