Microsoft hosted multiple events for the reveal of Halo 5: Guardians beta and plenty of gameplay footage have escaped those events.
There are two videos below, one was released by 343 Industries showing gameplay from all the weapons on all of the available maps. The other is a commentated, eSports-style match that really puts the new multiplayer changes in the spotlight.
There are two major changes in Halo 5 multiplayer. The first is the addition of Thrusters, which let players dodge into any direction, hover in mid-air and provide a short speed boost. The second is what 343 is calling the “smart scope” which provides an ADS-like scope on some weapons, and doesn’t come with a movement penalty.