I believe skills unlock on the first skill increase (in the main skill i.e. handgun) after hitting the required professional standing.
Although this is just my understanding based on reading and not a direct observation of MMS unlocking.
An alternative theory is that you are eligable to recieve a MMS skill increase after hitting lvl 10 profession standing and you see the unlock when you recieve the first such increase
Fire a few more rounds off I'm sure it will come soon
EDIT: I looked back at a screenie of my MMS unlock and it went...
HG skill increase
MMS unlock
MMS skill increase
so that could support either hypothesis