[?] Weapon Model Editing

[Insanitypays]

I keep seeing all these mods that have parts of weapons on others (such as a laser dot sight on an m16), Could anybody tell me how they do this, I have max 9 and Im learning how to use it if that is involved, thanks in advance, ECHO

-also, could this have something to do with giving a weapon 2d sights, I would like to know how to do that if anybody could tell me, but the above is priority one