All challenges considered, we live in a golden age of firearms. We have never seen so many options as what we have right now. And tomorrow, this statement will be obsolete because there will be ever more developments and new twists on old themes. How those firearms are manufactured, however, is a debate as old as the manufacturing process itself. What is better? Milled or stamped? Forged or cast? The latter debate has always been fierce. I am not an engineer. But I am a machinist by training and a gun owner by circumstance and I have a few things to say about whether you should choose a firearm that is made from forged or cast parts.
Forged and cast are definitions that tend to be thrown around indiscriminately. There is little thought to the process or whether it makes a difference operationally to whether a firearm will hold up under actual firing. The good news is that modern forging and casting processes are incredibly efficient at delivering a durable product, though the processes are different and worth noting for reasons that are beyond academic.
While casting and forging have been used for millennia, the latter makes us visualize the old blacksmith who beats glowing metal into tools on an anvil. This is how forging generally works. A furnace powered by coal (or wood if the latter is in short supply) and a supply of air rapidly heats up a billet, allowing it to be easily manipulated and beaten into shape, or pressed into a die. The final finishing of an ingot can be done with files, lathes, and mills. But other forging processes exist, like cold forging, which skips the heating process altogether.
The principal advantages of a forged part are strength and wear resistance.
Much of this has to do with the grain of the metal. Wood also has grain. If you sand it the wrong way, the surface will only become marred. But if you sand in the same direction as the grain, you will get a clean and smooth finish. The same is true for raw metal, like steel. If you clamp a piece of metal into a vise and go at it with a file, two things will happen: you will run along the grain and material will come off smoothly, or you go against the grain and you get scratching and chattering. When pulled from the ground and forged into shape, steel is hardened by the forces exerted against it but it retains that grain. Because the grain is left intact, the final piece is stronger overall and there are not any inherent weak spots to break. But an uneven heat treating can lead to defects.
Forged parts are characterized as having high tensile and fatigue strength over cast parts, although how much is up for debate. The most cited study on forged and cast part strength is also taken out of context. The University of Toledo concluded that forged steel had 26% tensile strength and 37% greater fatigue strength over cast iron—iron not steel.
Although most forging processes involve heating the metal alloy to make it more pliable, the original material does not change much as it never reaches melting point. Cast parts are made by bringing that same ingot to its melting point and dumping it into molds. The molten metal takes the shape of its mold and solidifies close to its final shape. Forged parts require some additional machining on a lathe or mill to reach their final shape. With cast parts, a manufacturer trades repetitive shop processes with the upfront expense of making molds. Some molds are 3D printed or made of wax and are destroyed but die molds can be reused continuously.
The advantages of casting over forging are not trivial, the most obvious of which are the flexibility of design and economical scale. There is no conceivable limitation on the size and shape of a given part with casting, while you are limited by your machine tools after the forging process. The casting process also allows the manufacturer to easily add alloys into the base metal to create a stronger product. Casting is valuable for large-scale operations, given that parts can be repetitively made quickly, but it is also handy for small-scale and hobby production. Forged parts ultimately involve the use of furnaces, forging presses, cutters, mills, and lathes. These are setups that have to be invested in that make small and specialty runs economically impractical.
On the whole, casting is the cheaper and more flexible option, but there are some notable disadvantages in terms of strength. The casting process, since it reduces the base metal from a solid to a liquid, obliterates the grain as it is reconstituted. The end result is a product that is more porous. There is also the risk of air bubbles becoming trapped in the cast as it hardens, creating faults.
Forged gun parts have definitive advantages over cast gun parts, and vice versa. It is just as easy to foul the heat treat on a forged part as it is to miss voids in cast parts. Proper quality control in forging operations and large X-Ray scanners serve to eliminate the worst possible outcomes from either part. From a purely technical standpoint, forged parts are better but it largely comes down to strength-to-weight.
Take Smith & Wesson and Ruger as examples. Smith & Wesson revolvers are made from forged parts, while Ruger pioneered investment casting and most of their handguns are made that way. Ruger wheelguns are thicker and heavier than their Smith counterparts. They have to be thicker in order to be as strong as forged guns. Likewise, some American AK rifles are coming from the factory with cast, rather than stamped or milled, receivers. Those cast AKs are heavier by a margin.
On the whole, weight is probably going to be the only disadvantage of a cast gun. If weight is of paramount concern, a forged gun will probably make you happier. But if that is off the table, there are plenty of great cast guns that are simply unavailable as forged. Keeping your mind open to both can only make a prospective gun owner happy.