What is the "Hammer Spur" on a Firearm or Gun?

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The term "hammer spur" refers to the end of a gun's hammer, shaped and designed to make it easy to cock the gun's action with the shooter's thumb. 

Hammer spurs come in many shapes and forms, but the function of a hammer spur is always the same: to aid in the cocking of the hammer in order to ready the gun for firing or to otherwise cycle the firearm's action.

The shape of the hammer spur was very important on guns with single-action trigger systems, since pulling back on the hammer is what cocked the gun for firing. The shape of the hammer spur was designed to make this easy, with a curve that contoured to the shape of a thumb, and often with lateral ridges to keep the thumb from slipping while the gun was being cocked. 

However, hammer spurs are not essential for some guns, such as semi-automatic pistols, and many manufacturers leave them off. Some shooters are tempted to cut off the hammer spur if it is not essential to the action of their gun, but experts caution against this, as the mass of the hammer spur is essential to the physics of the hammer striking the firing pin; cutting off the hammer spur may ruin the action of the gun.