The beta Gauss rifle is a test version of the scoped Gauss rifle that uses the model of a laser rifle and features a five round magazine. It only does about 25% of the damage of the final version, but has a 100% knock-down chance. It is only obtainable through console commands.
The beta version is especially interesting because, when fired, it shows the true trajectory of the Gauss rifle's shots, where as the "official" version of the weapon shows a misleading visual representation of the bullet trajectory. When firing the beta version, the round clearly fires quite a ways up and to the right of the center of the scope. For some reason this off-center trajectory remained unchanged in the "official" version, leading many players to confusing missed shots, because visually the round appears to be dead-center despite the fact that the actual coded path for the round remains up and to the right. A side-by-side comparison of both Gauss rifles, the beta and the "official", tested on multiple targets from multiple angles seems to confirm this.