5.56 has a longer throat for heavier bullets and case are thicker walled giving it a higher std rateing for pressuse
When I've weighed groups of 20 cases and taken the average, I've found Lake City 5.56 ammo to be one of the lighter cases (W-W Super headstamped cases are lightest, and Federal marginally lighter than L-C). Winchester, PMC and Sellier & Bellot were the heaviest in my sample. R-P was a little heavier than Lake City.
Thought it kind of odd that there was such a weight difference betwixt W-W Super and Winchester headstamped cases.
I haven't cross-sectioned any cases. Maybe L-C are thicker walled and thinner in the case head area to explain the weight difference.