M855/ss109 was designed to be heavier (longer range) without adding bullet length. They just added a steal insert which weighed more than an equal volume of lead. No specific AP properties.
??? Lead (Pb) is about 4x denser than steel (mostly iron, Fe) so if they did add a steel insert, it was not to add weight! Suspect that was the penetration issue I read many moons ago that got the insert in there.
Great thread..........
SS-109 is Fabrique Nationale's (FN's) name for their 61.5 grain bullet with the steel penetrator in the nose and what they call rounds loaded with this bullet.
No mention of the intent to be heavier with no size increase (which is not possible trading lead for steel) but to add a penetrator.