The AER9 was actually not the top laser rifle in service at the time of the Great War. The model line went up to the state-of-the-art AER12, which saw service in a handful of specialty units. The reason that the AER9 is much more commonly found is that it was much sturdier and more reliable than the models that followed. A new model, the AER14, was in development prior to the war, with at least one working prototype surviving in Vault 22.
The AER9 features a titanium housed crystal array which proved to withstand long years of exposure to the elements much better than the gold alloy housing of the later models. As a result, the crystal arrays stayed focused within operating parameters, rather than falling completely out of focus like the newer models. Power is drawn from microfusion cells and processed through a wave/particle diverter manufactured by General Atomics International. The diverters are protected by a carbon-fiber housing, preventing frequent malfunction, but when a diverter fails the weapon becomes unusable.
The focused laser rifle is an improved laser gun, that uses microfusion cells as ammunition. Dealing a minimum of 12 damage per attack, it has the second lowest damage output for an energy rifle in the game.