There are DOZENS of different references, depending on the make and model. Some use a serial number. Some, like Remington, use a barrel letter code. Some, like Mossbergs, never HAD a serial number- so we have to go by years that THAT model was made.
But ONE website for ALL guns ? HAH! You WISH!
However, if you can post info on whatcha got, we can make a stab at it. More info= better chance of getting an answer.