when I try to upgrade my weapons, I see 'the following weapons are available in the current city' but then when I sell my weapon to buy a different one and attempt to upgrade, the only ones available are the weaker ones.

different tech levels have different weapons
go to the high tech level cities in order to find the good weapons.

what I mean is, the bigger weapons do show up as available in the current city, when I have an existing weapon. When I sell my current weapon to upgrade (I'm assuming you can only hold one at a time) they are no longer there.

I think I found it, need a vehicle big enough to carry the weapon.