I Thought This Was Settled...

David Codrea brings up an old discussion on what would be used by modern hunters on a T-Rex, though I suppose self defense shootings would likely be necessary for anyone living in an inhabited area.

I thought this matter was already settled by Lara Croft. You simply have to find a large clearing with four ancient spiked wheels in it, shoot it with dual .45 ACP 1911-A1s, or Dual .45 ACP USP Expert Match handguns, to enrage it. Run towards one of the wheels, and when it charges, use your special "bullet time" ability (like in John Woo movies) to stun it with a single headshot, and it will slam into the wheel. Do this with three of the four spiked wheels, and the beast will fall.