How can I upgrade my weapons to level 10?
The description of the Gunsmith achievement seems to indicate that you can upgrade your weapons to level 10.
So far I can only upgrade my weapons to level 5, no higher upgrades are available to me. Do they appear later in the story or do I have to do something else to get them?
To get to level 10 with a weapon, you need to complete the game then import your character on a new playthrough, where you will be able to buy/collect the remaining upgrades.
You can upgrade weapons to level 7 at the start of new game plus and you will get 3 automatic levels upon collecting the weapon again in the next playthrough.