Final answer:
The three main parts of a modern firearm are the action, which handles ammunition; the stock, serving as the handle; and the barrel, through which the projectile travels upon being fired.
Step-by-step explanation:
The three main parts found in a modern firearm are the action, the stock, and the barrel. The action is the part of the firearm that loads, fires, and ejects the ammunition. The stock serves as the handle of the firearm; for rifles and shotguns, it is the part that rests against the shooter's shoulder. The barrel is the tube that the projectile travels through upon being fired, and its design affects the accuracy and velocity of the shot.