When you equip a weapon level that is higher than set character level, it suffers a penalty. Has lower damage, slower fire rate, and lower magazine cap.

Rules are simple:

1. If your character is higher than weapon's level, there is no weapon penalty.
2. If your character is lower than weapon's level, there is weapon penalty.

Still with me? Look at the table in the next secton.

As you can see, a level 2 character using level 3 or higher weapon, the latter will suffer a penalty, that is, that level 3 weapon or higher will work as if it is level 1 weapon. Same goes for the level 3 character using level 4 or higher weapon, the result is that weapon will have stats like of level 2 weapon.

For level 50 character using level 49 or lower, of course, there is no penalty. (see rules in "Overview" section.

So what will you do now? Will you still use level 49 weapon when playing at Abyss 15 (recommended level 44) for example? Is level 49 Thunder Raptor will still be good?

The answer is, yes. But using same level weapon is recommended. However, due to the fact that rare tag weapon is good enough, with legendary parts like Burst 3, Gluttony and Autoloader, it doesn't matter. I did several runs using level 40+ weapons against lower Abyss and still the penalty is not that of a big deal.

However, don't use level 50 weapons (even if rare tag) to play at lower level character (49 and below) because as you can see, the penalty (according to the table) is huge (-10 levels and more!).

But where did I get the -10 level calculation, you wonder? Look at level 49 character column. You can see that the next column after it shows penalty at 40. So if you are using level 50 weapon when you are level 49, then the weapon will work as if it is level 40. So simple math: 50 - 40 = 10. Now look at level 3 character. What did you see? Level 50 weapon will work like level 2 weapon. That is 48 level downgrade.

Now, if you wanna play at level 1, is level 50 weapon matters? Not at all. Coz it will work like level 1. So you don't have to use another level 1 weapon (credit to Nivea for the idea).

:)

1.)
Official guide: (Japanese version) [kouryakutsushin.com]

2,)
Drae_perin, Nivea and Yuna5422 who helped me understand the weapons penalties