Each weapon in the game has a weight level. You get weight levels because when your on a mission you can only carry so much weight. You can own as much equipment as you want but can only carry so much on each mission. Below describes the weight levels.
weight level 1/4 = you can carry up to 24 weapons of weight level 1/4 and no other weapons. weight level 1/2 = you can carry up to 12 weapons of weight level 1/2 and no other weapons. weight level 1 = you can carry up to 6 weapons of weight level 1 and no other weapons weight level 2 = you can carry up to 3 weapon of weight level 2 and no other weapons. weight level 3 = you can carry up to 2 weapon of weight level 2 and no other weapons. Weight level 4 = you can carry up to 1 weapon of weight level 4 and certain combonations of weapons of weight levels 2 and below. Weight level 5= you can carry up to 1 weapon of weight level 5 and certain combonations of weapons of weight levels 1 and below. Weight level 6= you can carry up to 1 weapon of weight level 6 and no other weapons.
In case you want to carry weapons with different weight levels I will discribe how it works. Your character has a maximum weight level of 6 (and 8 with a back pack) when you combine weight levels. So if you want to have 2 weapons of weight level 1 and one of weight level 2, you could. To find out how much you can carry you add the weapons weight levels toghther. here is an example
WL1(weight level 1)+ WL1+WL2+WL2=WL6
I got 6 by adding the weapons weight level. I think you understand weight levels enough now so go buy some weapons.