I don't think there is an all inclusive site for ALL weapons.
There are a few good books out there that have an index listed by country, but again they're not all inclusive.
When you think about all the guns made by every country, the list gets pretty big, and then you have the one model of gun made by "X" amount of countries, like the AK, you've got Russian, Chinese, Yugoslavian, etc. etc. it would get kind of crazy.
Is there anything in particular you're trying to ID?