I think what he is getting at is a 55 gr. .223 projectile has 1099 ft. lbs of energy, vs a 69 gr. projectile has around 1379 ft. lbs of energy. That's almost 300 ft. lbs. of energy more which translates into more knock down power. 300 ft. lbs of more energy is nothing to sneeze at. The bigger the bullet the more knock down power. However the speed of the AR15 projectiles produce massive tissue wounds so one kind of crosses out the other. More speed with more tissue damage or less speed with more ft. lbs of energy. Either way you go it's going to be a mess for the guy on the other end. I believe bullet selection is more important than weight or speed when dealing with the AR platform. Unless you are shooting a AR10 .308 with a OTM projectile and then it wins hands down. Just saying! I just used .223 ballistics because that's the chart I had on hand. I am sure the 5.56 has similar effects.