To get max weapons and unlocks for your kit in Battle of Stalingrad multiplayer, you need to unlock them in the single player game.
That can mean up to 80 hours of mind numbing single player boringness if you want all weapons and skins for all 8 aircraft.
Here is how you can unlock all weapons and skins with minimum pain.
1. Choose your aircraft and choose a LONG mission
2. Set autopilot ON (A-key) and accelerated time to 2X
3. Go take a dump, read a book, watch a TV show, snog your girlfriend/boyfriend
4. Return, cash in the XP and repeat!
Of course there is a small risk your machine will be shot down or crash into something/someone but for me this workaround works about 4 out of 5 times. I have now unlocked all the weapons and skins on the FW 190 and Bf109G.
PS, this 'cheat' is officially endorsed. I read about it in a post from 'Zak' who is the community manager on the devs official website!