How to get a -B gun?

Ok, I noticed that I saw some players use a gun called "SG552-B" and in GMs/MODs weapons owned, I see AWM-B. They all have AMMO like this:
AWM-B:
10/20
SG552-B:
35/70
Aug-A1-B[I know how to get]:
35/70
Can you explain how to get one a -B weapon, not the temp weapons that says ?

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