Getting all weapons for 100% completion

In the menu's progress section says I need to get 64 weapons in order to achieve 100% completion, but I have no idea where to get all of them. The shop only sells half of them and online guides don't provide any comprehensive info about it either.
Anyone already did this and can help me?
Thanks in advance! :)
Edit: Locked or unlocked, there aren't 64 weapons on the shop. That's the problem.
Best Answer
I'm not going to go into detail here on how to unlock all the weapons, but they should all be accessible through the shop.
Any weapon that is locked should tell you what you need to get it when you hover over it.
Most will be getting to a certain part in the game, the best ones (the ones under the special category) require a certain amount of a particular event or task to be completed.
There are a few other guns in the game, through DLC, but these should automatically unlock during completion of the said DLC.
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