The supposed damage caused by steel-cased ammo is one of the multitude of myths that has been spread for years among the gunning community that has absolutely no bearing in fact. (Dare I intone one that seems a particular bugaboo -- WD-40, which I used almost exclusively on my guns for decades and has never shown a single sign of causing the slightest harm -- but then, I don't soak my guns with ANY gun oil). Tulammo is not necessarily the best ammo out there, and tends to be pretty sooty, but it will not hurt your gun. If it feeds, use it. Just because "my brother-in-law" or "the guy at the gun store" says it -- or 10,000 yahoos online, for that matter, some of whom I suspect have never seen a gun outside of the movies -- does not make it true.
Hey, in Russia there are still thousands who'll swear Stalin was a saint...