I am considering the possibility, if upgrading exists currently, that the gun only says the very next clue that you need.
If there is truth to this, we would need to get the gun after we've done whatever triggers (most of) the cues, then we should get the last cue or the 'upgrade complete' cue. Then it would be done, or be able to be PaP'd.
Upgrade complete may not change the gun, it may change the PaP to accept it.
Shrug. There must be a way, or they're fixing it to return when able. It can't be unupgradeable, i hate Trollarch too much to hate them more, it's a psychological impossibility.
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Treyarch, please don't shit the bed once again. You've been in the business too long to act this novice, and yet, here you are. Aren't you guys tired of the shame? Think of your families; do you really want them to have to wear bags on their heads in public?
Sigh. Happy gaming?