Noun (mān′sprĭng′) or (ˈmeɪnˌsprɪŋ)
The spring that powers a handgun's firing mechanism. The mainspring is that spring on (or in) a gun which does the job of causing the gun to fire. That is, while other springs are often used and are also integral, the mainspring is truly the main spring in that it performs the most important function.
The mainspring is the spring which causes a gun's hammer and/or firing pinto fall and/or travel forward under spring-exerted pressure.
On guns which lack a hammer, such as striker-fired pistols and bolt action rifles, the spring which drives the firing pin is often simply called the firing pin spring, which is correct - but it can also be correctly called the mainspring.
Revolver MainspringPistol Mainspring