In the trailers and the movie we see the Vengeance prepare to fire on the Enterprise, and two railgun-like things come out of the... saucer ? Does anybody know what they are and where they are on the ship, exactly ?
They're photon torpedo launchers, and they emerge from the hatches on the underside of the saucer. Like giant-sized versions of the ones on the Kelvin.
Wasn't the Vengeance facing Enterprise at that point in the movie ? The hatches are on either side of the saucer, so how can two of them be side-by-side like that ? I've only seen the movie once, mind you. EDIT: In fact, if you look at pictures of those "gun" (and torpedo launchers is quite plausible, though we never see them fire, I think) they seem to be in a depression in the hull next to an outcropping, so I thought they were forward facing.
Having watched the movie a second time, I seemed to hear Marcus say "lock aft phasers", and the Vengeance seemed to be side-by-side with the Enterprise at that point. My guess is that they came from the underside of the saucer (there's a space port and aft of the saucer with space there but no doors).