from looking at the trailer again:
The "pistol" the chief uses might not be a pistol at all. if you look, he fires one shot and something blue explodes in front of him. later when he puts the "silencer" on, if you look he doesn't screw it on. he appears to slid it on to some form of rod on the front of the gun. My theory is some form of explosive flare gun.
also, maybe chief got an upgrade to his MK. 6 over the course of halo 3 that wasn't essential to the story that allows him to have a built in jet-pack? idk but it was cool ^.^
Sry for double posting, but I think it's a grenade launcher, it isn't a "barrel" at all, its the exact same shape as the one fire before