Go up levels
You cant.
get to level 48 and you unlock the ACR
Level 48
Uh....no
when you are level 4 you unlock the intervention and 50 cal.
You cant.
In MW2, online and offline play do not cross over. You either complete challenges and gain levels in one or the other (at one time). Hence, you cannot unlock online weapons by playing offline.
No, not legally, you can "hack" the game, but beware you can easily be banned for this.
get to level 48 and you unlock the ACR
no
you get access to the Mw2 museum. three rooms with characters and weapons from each mission.
Level 48
get 2500 headshots
level 4
Unlock all attachments.
lvl 34
Uh....no