Q. What is SS-109? Is it the same as M855?
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. (FN calls M193-type ammo "SS-92.") The US military's M855 round is loaded with the SS-109 bullet, though the US military has additional specifications that ammo must meet before it can be called M855. So, while all M855 is loaded with SS-109 bullets, all "SS-109 ammo" will not meet the M855 specs. For example, the British purposely underloaded some lots of their ammo in an effort to get their L85A1 (SA80) rifles to cycle properly. The ammo is still loaded with SS-109 bullets and labeled as SS-109, but it is nowhere near the M855 velocity specifications.
Diagram of M855 construction.