Greetings Ark Hunter,
Thank you for contacting us regarding Defiance.
I do apologize for any confusion caused when trying to increase your weapon skill with a specific weapon that is already max level. The experience for a specific weapon type(Assault Rifles, Infectors, Pump Shotguns) can only be increased by the total amount of experience available to the specific weapon you are using. Once this specific weapon reaches it's max level it will no longer provide experience to the overall weapon skill but some weapons will provide combat bonuses for having achieved max level. You can locate this by opening your Inventory window and selecting the appropriate weapon. Once you have selected the weapon in question you will find the experience bar in the stat readout for the weapon on the right side of the window. Your weapon skill will increase again once you equip a non-max level weapon of the same type.
If you have any additional questions or concerns, please don't hesitate to let us know. Thank you for your continued support of Trion Worlds and for playing Defiance.