Or more specifically, from the above mentioned page;
Q. Is Wolf-brand (or other steel-cased) .223 ammo okay to shoot in my AR15?
Well, if you didn't start a flame war with the M193 v. M855 question, you have now.
Many former Eastern-Bloc countries use steel ammo casings in place of brass, as the cost of steel is much less. The steel casings would quickly rust if left untreated, so one of two methods is used to treat the steel cases: lacquer or "copper-washing." Most cases are coated with a protective lacquer, usually green or gray in color, which is the cheapest solution. Copper-washed ammo is a blotchy bronze color and usually the entire loaded round is coated. Copper-washed ammo is usually only found in the East-Bloc calibers. The lacquered cases generally work without problems in most calibers, but 5.56/.223 is often an exception. Unlike the Soviet-designed cases, the 5.56 cartridge has very little taper to the cases and its length to diameter ratio is very large. The result of this is that 5.56 ammo has more friction during cartridge extraction and comparatively less extractor surface area. This usually isn't a problem with brass cases, but when lacquer is factored in, stoppages often occur.
Often the lacquer on the cases is not evenly applied and visible drips and runs can be seen. When the chamber of the gun heats up from firing, the lacquer will often melt and coat the surface of the chamber, in effect gluing the case into the chamber. In some cases, the extractor will be able to remove the case (though wear on the extractor is increased), while in others, the extractor will slip off the case or pull through the rim. When this happens, not only does it cause a stoppage, but the case often has to be removed by inserting a cleaning rod down the bore and pounding out the case. Then the lacquer must be scrubbed out of the chamber, which is not an easy process.
To be sure, some people report no problems using lacquered ammo and enjoy the fact that it is available at very low cost. Nevertheless, enough people have problems with it sooner or later that it may be advisable to avoid lacquered 5.56 ammo altogether.