A handguard assembly for a firearm includes a stationary handguard portion, a movable handguard portion, a catch, and a latch mechanism. The catch is positioned on an interior of the handguard assembly, and the latch mechanism is position on an exterior of the handguard assembly. Both the catch and the latch mechanism maintain the movable handguard portion in a closed position. A STANAG mount adapter assembly includes a body portion, two flange portions, and an alignment mechanism. The flange portions are positioned on opposite lateral sides of the body portion and are configured to engage a STANAG mount, while the alignment adjustment mechanism is configured to adjust the lateral alignment of the body portion with reference to the STANAG mount. The STANAG mount adapter assembly can be used together with the handguard assembly to create an elongated rail platform on an SA80 rifle.