I was playing the campaign as brownpants when I brought a cargo truck back, and unlocked the L85A2 in the armory. It was there for quite a while, when suddenly I loaded up the campaign and it was gone. I'm not sure if Its suppose to disappear, but when I get a new cargo truck it goes right on to the next unlock-able weapon and i'm not sure if I can ever get it back. Like I unlocked the m72 and the Deagle, but the L85 just disappeared from existence suddenly from my armory, even though it was there the last session, the next one I loaded up it was not there.

EDIT:

I just noticed the "out of stock" message that came up with the m79, For some reason that message did not pop up with the L85.