完成單人和雙人合作模式後就應該會解開所有武器了

另外我還在懷疑需不需要撿槍這動作來解開武器
印象中我玩雙人合作模式都沒撿過槍......

我在其他論壇有注意過, 有人說完成特定關卡就會自動解出來

相關論壇
http://www.xbox360achievements.org/forum/showthread.php?t=222938

There are a total of 17 weapons to collect. You gain these by playing and completing story based missions. In single player, you will unlock all of the associated weapons upon finishing the level Michigan Ave. To unlock the rest, play missions 1-3 of cooperative story mode. There is no worry of missing a weapon in the game as long as you complete the levels. You do not have to pick weapons from dead enemies to gain them.

意思就是不用擔心會遺漏武器, 只要完成單人故事和雙人故事就OK了
特別注意紅字, 「不用撿武器」
  

P.S.  兩種AK都要玩雙人合作才有, 單人那個回憶章節雖然有AK, 但別被誤導