Originally Posted by
Dtrick
If you look under the gun icon you will see --0- or ----. This means that a weapon can have mod slots added to it. The 0 means that the slot is already open. So a weapon with all mod slots open would have 0000 under it. First slot is stock, second barrel, third clip and fourth is scope. So a weapon that is -00- has the barrel and clip mod slot open on it.
For weapons that can not have mods, there is nothing under the icon. Also, if you click modify weapon, it states that there are no actions for this weapon.