Well, if you're talking about 1200ft-lbs at impact, then your 72gr bullet is only good if you shoot the deer at less than 5 yards or so. A 75gr bullet at 2700fps (this is the max listed velocity for .223Rem for 75 grain bullets in my Hornady 6th edition manual) has 1,214ft-lbs of energy AT THE MUZZLE. Given that a .224cal spitzer point boattail bullet at 75grains has a BC of roughly .400, by the time that bullet reaches 10 yards from the muzzle the energy has dropped to 1,189ft-lbs. At 100 yards the energy is down to 1,019ft-lbs.

A .243Win with a 100gr bullet will easily reach a MV of 2,900fps in standard length barrels. This yeilds a muzzle energy of 1,867ft-lbs. At 220 yards the 6mm bullets still (at a BC of .355) retains 1,216ft-lbs of energy, matching the energy of the best the .223 has to offer at its muzzle.

Mike