Powerhouse achievement bug?

[Dani]

So I was going for all achievements and the last one was Powerhouse, which I was supposed to have all available weapons in my inventory in one save.

Done that:
https://imgur.com/a/efy79c2

But the achievement didn't pop up for me. Then, I tried father Google... Useless. But, every weapon guide I saw mentioned I'd get the double barrel shotgun and the flamethrower on chapter 13... I got both on chapter 11 (no bugs, just went off of the main quest and got back to Sykes before chapter 13).

Does anyone know what went wrong for me? Did I miss something?

Thanks in advance.