In regards to the biggest gun on any gun truck, you have to think through the application: counter ambush against forces with only light cover and spread out. For light brush, you need saturation fire. In Iraq, where you have thick concrete walls and buildings, precise direct fire with a punch is needed. So, in Vietnam, an M2HB is as heavy as you needed. Granted, as many as you could get. The M134 was the best, but they could only be obtained from down choppers, so hard to get.
Back to the You Tube "video", it is in fact a slideshow. That's not a criticism, it's a correction of terminology. Get over it. FIDO.