as you should know, that dlc is on the house on origin right now, but when i wanted to unlock the new guns by playing, i get no progression, as if i dont have the dlc at all.
please note i only unlocked the dlc on origin and did not downloaded the maps, but that by all means should not prevent me to get the new guns…
I think you actually need to play a map of Second Assault DLC beforehand.
If u just unlocked it its not installed yet so the game does not know you own that content 
May the Force be with you
I think you need to download and install the DLC, which would allow you to start unlocking Second Assault weapons and doing the assignments.
Anyway some guns like the F2000 need you to play on SA maps (Metro for F2000) so you’ll need to download the entire thing nevertheless.
Just unlocked the f2000 and it is a beast. Took me like 10 minutes on one of those 24/7 metro servers to get.
You have to download the maps first and then change the browser options to second assault maps. Then you have to play on the second assault maps for to unlock the guns, some of them are very easy to get.
You have to have the actual content genius.