A firearm and a kit for modifying a firearm is provided. The firearm has an upper receiver, a lower receiver, and a releasable magazine that releases when the upper receiver rotates away from the lower receiver. The firearm further has a firing pin with a striking end that has a cylindrical shape and a substantially sharp shoulder. A hammer is also provided with a depression disposed in a front surface thereof, the front surface configured to strike the striking end of the firing pin when the upper receiver is next to the lower receiver, and the depression configured to align with the striking end of the firing pin when the upper receiver rotates away from the lower receiver.