Creating guns without any permissions

İf you know Payday 2 game, there is every gun real but their names got changed.Why they done that? They can name them by their real names.if you say’they didnt took any permissions to make them so they are changed names’ its like a copy and its understandable which gun is this.Do I have to pay and get permissions from creators of these guns if I do so?

Weapons are often trademarked and copyrighted

so ?? may you explain it more please.

There is a GDC series of videos dealing with copyright issues. For example: Practical IP Law for Indie Developers 301: Plain Scary Edition - YouTube

I am not a lawyer but it seems like copyright enforcement is up to the owner. Meaning a manufacturer that owns copyrights to a weapon or item can choose to let people blatantly infringe on his IP. Then decide to sue only one party of his choice at anytime all the while ignoring the rest. So it is never safe to have copyrighted items in your game even if other people are doing the same.

Or atleast that is my understanding. You should talk to a real lawyer.