My wife an I are also fans of SOG, but strictly for its entertainment value. The show too often strains credibility, and in the episode of the 137 unsuppressed .50, and the suppressed 118, throws it out the window. If those numbers were supposed to be dB, one must question the accuracy and/or methods of their measurements. But it is mostly fun and some neat stuff.
Back to the why, or more properly, "why not" of totally integrally suppressed 500 magnum. The bore is simply too large to "hide" in a bull-barreled sized outer tube. I certainly believe it could be done, but unless someone can prove differently, I also believe the outer tube would have to be at least an inch and a half for effective suppression. Limited to subsonic rounds, it might be possible to go a little smaller, but the bulk and weight would still be considerable. Perforating the barrel to bleed off firing pressure would easy, attempting to turn down the barrel to remove weight would not be anything I would advise or attempt.