SM for Powered Assault Commando.
The point of a lexicon is to record terms and acronyms that already exist in common usage. Just making things up is fun but not really useful.
Actually, that is fine, as that is what a lot of members used to/still use to call a Powered Assault Commando. I suppose it isn't a "Term", so to speak, but this lexicon is only for common ONLINE talk.
Such as "Lol", "AFoL", etc.
BM is a common euphemism/acronym for "Bowel Movement" while SM just looks like a short form of S&M which stands for "Sadism and Masochism".
BM isn't commonly used -Well, I shouldn't say that- But from experience, that would be used just as much as SM(Powered Assault Commando); if not, less. On the note of your S&M remark, it can mean mutliple things. Such as: Powered Assault Commando, what you said, Slave and Master(adult..."play", so as not to mention what so many people are immature with), a Metallica album, amongst a quantum of other things.