http://en.wikipedia.org/wiki/Forging

In a nutshell, forging forces the metal into a shape, whereas casting pours it into a shape. Due to the nature of metal and it's molecular grain structure, forging typically yeilds a stronger part.

Casting simply pours the metal into a mold.. not necessarily bad, but often there's more porosity with casting, a random grain structure, and thus a overall weaker part.

As for a forged slide starting from a casting, it's possible. A manufacturer could start with a rough casting and forge it into it's final shape. But I would think that in the end, it still wouldn't be a strong as a slide that started life as a rolled billet.

These are all rough generalizations - there are so many variables that can affect the end strength of a part, like the alloy used, heat treating and annealing, machining steps, and so on. I'd suggest doing some Googling on metallurgy, steel, etc.

When I was at Smith & Wesson all gun parts stanrted out as round steel bars, cold rolled. The frames were forged, other parts milled.

In forging, the billet is placed into a die and hammered to shape. Forging is supposedly the best method for developing the grain in steel.

Casting is simply pouring molten metal into a mold, the same as making candles or popsicles. Its economical, efficient, but not for the best of parts/finishes.

Bill Ruger learned of investment casting during World War II. In investment casting, a wax model is used to form a ceramic mold. Molten steel is poured in, melting the wax which escapes through a vent. This forms a vacuum in the mold and arranges the grain structure very much like forging.
Investment casting produces steels nearly the equal of forging, and less expense.

Probably told you waaay more than you wanted to know.

Bob Wright

P.S. Casting or forging are only first steps, milling, or machining, steps are required to finish the part.