UPDATE: (borrowed from JJR0906's solution)
On the latest patch you will now have to buy all previous levels of weapons in order to earn XP for the newest one. So, for example, you can only start only earning XP towards a level 3 gun, once you've purchased the level 2. Therefore you now have to buy all five of the level guns to get this achievement
Things to still note after the patch:
If a gun only has 4 levels, you just need to buy the 4th level, but you do not need level up gun to 5. Like the double barrel. I bought the 4th level, and never leveled up to 5. It still unlocked. Same for grenade launcher. Its only level one. I never leveled it past that
Also, if a gun has TWO level 5 versions, you have to buy BOTH level 5 versions.
There is a lot of confusion about this one on the console version.
I wasted at least 15-20 hours extra on this one that I didn't need to.
You have to level your weapons up, but you do NOT have to buy ALL levels!!! You have to buy the LAST level only. I upgraded a lot a of guns I didn't want or use thinking I needed to upgrade level 1 through 5. Also the highest rated guide on the console version says if a gun has 2 rows "one row" needs to be bought out. This is WRONG. Only the level 5 COLUMN needs to be bought.