To fully upgrade a weapon you need to reach at least level 15 to unlock the three upgrade levels; you must also unlock the signature ability for your chosen weapon. Unlike the vehicles, there is less need trophy wise to get these signature abilities and some of them can be a little annoying. The Idol Pistol available at Friendly Fire has a super easy ability unlock if you're looking for an easy one.

Once you do, head to any Friendly Fire and buy the upgrade kit 3 times, this will cost you a total of $130,000. Once you've bought all three upgrades and unlocked the signature ability, this will pop.