“Except for the barrel, most parts of most guns could be made of parts that could be made from castings of brass I suspect.”
No
The original gunmetal was a variety of bronze, an alloy of copper, tin, and zinc.
4140 steel, also called ordinance steel, has 0.4% carbon and chromium, manganese, and other metals in smaller quantities. 4140 steel is a hard and durable steel used in many gun components such as barrels, bolts, and receivers.
While most gun components that experience significant forces are made of steel, some parts may be made of anodized aluminum alloys. The most widely used aluminum alloy is 6061 aluminum, also called aircraft aluminum, which contains magnesium and silicon. 6061 aluminum is often used for secondary chambers and suppressors. The stronger 7075 aluminum alloy is used in some receivers — the AR‑15, for example.
Receivers are typically made of steel or aluminum, and they are manufactured through stamping, machining, or forging.
In modern repeating action firearms, the action is the mechanism that loads a cartridge from the magazine, fires it, extracts the casing from the chamber, and ejects it from the gun....most often made of carbon steel, stainless steel, and spring steel.
The sear holds back the hammer or bolt and, therefore, the firing pin, until the trigger is activated.
“agricultural parts like plow shares, cultivator teeth, and disc blades”
“automotive parts including shafts, pinions, and gears”
https://wisconsinmetaltech.com/4140-steel-applications-top-10/