it used to be prior to phantom liberty and the 2.0 update that we could upgrade all weapons(clothes no longer matter, so not talking about them here.) doesn't seem to be any rhyme or reason to it either. i can upgrade the sniper you get from panam and i can upgrade a legendary defender, but i can't upgrade Rebecca's shotgun now. i even crafted a throwing knife for a mission where i needed to be stealthy, but couldn't upgrade it at all. makes the upgrade tab pointless now.

You can't upgrade, only craft new, because, um...
(overly enthusiastic:) Crafting everything is awesome!

that is perhaps the lamest change they could have done. there is no iconic lmg, at least none that i can find. idk if they added one with phantom liberty or not. haven't encountered one yet if they did. so now my defender will just fall further and further behind?

Yea its stupid. Granted the crafting was made way too important before but now they went to the otherway and made it pretty much useless.