Improvements and modifications in the rifle and submachine gun platforms to improve accuracy, improve user functionality, simplify and improve the machining and manufacturing and assembly process are presented. Specifically, the disclosure describes a hinge pin connection system for an access door, a retention mechanism that eliminates freedom of movement for hand guards, cuts in a barrel extension that reduce or eliminate barrel rotation, a barrel retention mechanism that allows for securing of a barrel from the sides of a firearm, a dove tail connection between an up rod and bolt carrier, and an aperture located on a lower receiver for mounting slings and other accessories.