But I will bet that the frame, slide, and many internals are cast steel
.010 inch under that nice Blued steel, could be a complete void, it's called porosity and it's fairly common.
There is a huge difference between vacuum furnace, investment cast steel, and die cast Aluminum. Ruger is one of the world leaders in modern investment casting. The receiver for the Model 77 bolt action rifle is investment cast, as is the Mini 14 receiver, and the Ruger #1 Rifle, considered to be one of the strongest actions on the market today. They are every bit as strong as any receiver that is milled from solid. Porosity is all but non existent in modern steel, vacuum investment cast furnaces.
The same is not true in Aluminum die casting. If an Aluminum Die Cast Mold is not properly vented, or if it doesn't include enough overflows, or if the overflows are not large enough, or if the shot pressure is not high enough, or if the shot pressure is too high, or if the metal temperature is not hot enough, or if the metal it too hot, porosity, as well as voids, short fills, and crystallization can and do occur. That's way too many "if's" for something that needs to be as durable as a slide on an auto pistol.
As was pointed out, for the price of the Walther P-22, (mid $300.00 range), there is no reason the slide should not be made of steel. There is just zero reason for a slide to fail like that on a .22 rimfire.
The Ruger .22 Auto frames are actually made from stamped steel, much like the receiver of an AK-47, and have been for almost as long. No one questions the durability of an AK. The bolts are turned from steel bar stock. And in spite of the steel construction, the street price of the standard models of the Ruger Mark III's are in the low to mid $200.00 range. I actually liked the Walther, and for a time was considering buying one. But the performance was just too spotty for a pistol in that price range. You shouldn't have to gamble when you buy a .22 for over $300.00 in today's market, and or economy. Bill T.