An upper receiver for a weapon, the upper receiver having a plurality channels each being oriented in a first direction and wherein each of the plurality of channels intersect an elongated channel extending in a second direction; a modular rail having a pair of securement features configured to be slidably received within a pair of the plurality of channels such that modular rail can slide in the pair of the plurality of channels in the first direction until the pair of features can slide within the elongated channel in the second direction.