Steel case ammunition has been widely used throughout Europe for over a century. The Russian military, in particular, as did all of the Warsaw Pact countries, used steel case ammunition almost exclusively. It is cheaper to make and military ammo is, for the most part, disposable goods. Germany also made widespread use of steel cased ammo in rifles and pistols during WW-II. The U.S. Lake City Ammunition Plant experimented with steel case 5.56 ammo around 1967-1968 for deployment in Vietnam. It was scrapped because of functionality issues.
Nearly all European designs will run with steel case ammunition for that reason. Look at European military ammunition design and they're nearly all slightly tapered. Steel cases do not expand and contract at the same rate brass cases do. The AK, for example, was designed around a well tapered round that functions virtually flawlessly. The AR platform was designed around a straight wall bottleneck round, the 5.56. As a result, a steel case not contracting back fast enough will muck up the firing cycle with gas pressures against the bolt carrier group. It also tends not to release itself from the chamber as smoothly as brass. It's fine for practice where a FTE is not critical.
As for steel bullets, those too, have been around for well over a century. Mild steel bullets are much cheaper to make by the billions than copper, and doesn't deplete a more valuable war material. However, if you look at European military rifle and SMG barrels, the rifling tends to be deeper and wider than on U.S. rifles. This offsets the barrel wear of steel to steel, even though it is soft steel projectiles going through a hard steel barrel.
To a military, small arms barrels are a normal wear and tear item. Think of how many U.S. M1 Garands, 1903 Springfields, M14s and M16 that have been rebarrelled along the way. Germany rebarreled millions of their WW-I Mausers prior to and during WW-II, as did the USSR, and most notably, Finland (used Russian Mosin-Nagant rifle receivers with Finn barrels).
The Glock and steel ammo, well, in many of the countries where Glocks are used, steel case ammo is the norm, which is why Glock functions well with it. Also the type of polygonal rifling does not lend itself to the same type of wear that conventional cut rifling does.
And, with a few of the right tools from Brownells, and some patience, you can replace your own AR barrel - try that with an M14, Garand or Mauser without the right tooling and gauges.
