Is steel cased ammo bad for my AR?

For high-volume range blasting in an AR that likes it, steel is a reasonable money-saver — but with caveats. Steel does not seal the chamber or extract as smoothly as brass, runs dirtier, and the bi-metal jackets on some steel ammo accelerate barrel wear over very high round counts. Brass is cleaner, extracts better, is reloadable, and is gentler on the barrel and extractor. Run brass for carry, precision, and any gun you care about; save steel for cheap practice in a gun that runs it reliably.