Laz,
I may be reading your post wrong but I am hearing two possible misnomers. You said that DPMS states to stay away from steel cased ammo but then say your ammo is copper washed.
The copper washing is in reference to the bullets themselves (projectiles) not the cases. The steel cases whether they be lacquer coated, zinc coated, pick your coating, will not obturate (seal the chamber) as well as brass cases will. This may lead to premature throat erosion.
Some argue that by the time you shoot enough ammo to erode the throat of the barrel you will have saved enough on ammo to replace the barrel a couple times over.
Just wanted to clear things up a bit so that we are all on the same page when giving or receiving information to assist in this discussion.