The LMTA(R) Modular Ambidextrous Rifle System (MARSA(R)) lower receiver offers ambidextrous controls to include: selector, magazine release and bolt catch and release. The ambidextrous magazine and bolt catch mechanisms are machined directly into the receiver and have been designed to function as ergonomically as possible. Utilizing the LMTA(R) patented Monolithic Rail Platform, MRP(TM). The only true monolithic rail platform, milled from a single solid piece of aerospace aluminum forging. It utilizes two locking bolts accessible from the side of the receiver to lock in the barrel extension. This patented technology engages the barrel extension for a full 360 degrees around the extension and makes the barrel removable within seconds with a return to zero of the same barrel. One receiver can utilize numerous barrels of varying length, material, and caliber.