In embodiments, a modular rifle includes a lower receiver assembly, an upper receiver assembly housing a barrel, and a coupling mechanism. Upper receiver assembly has two portions, an aft portion and fore portion, commonly comprising a hand guard. The barrel is releasably attached to the upper receiver first portion by a hand-tightenable barrel nut sleeve connector. Upper receiver second portion is both releasably attached to the barrel nut connector sleeve and to the upper receiver first portion to prevent the barrel nut connector sleeve from rotating during firing of the rifle and to keep the second portion from detaching during firing of the rifle. Attachment of upper receiver second portion to the barrel nut connector sleeve is accomplished by a removable take-down pin and common screw clamp, which secure the barrel nut connector sleeve. Upper receiver first and second portions are connected by an attachment tab and receptacle.