It's a decent resource.
The problem is, he doesn't really list what countries the weapons are in service in. He'll talk about who built them and maybe mention in the history if someone bought large numbers of them. So, it's easy to figure out what China is using, because they make their own weapons. But some smaller countries import designs, so they are much harder to figure out. A google search can probably help you with them.
Also, part of the problem is that many countries like the US may use many weapons. I know the US military uses at least 4 different handguns. And I just read an article in one of the gun magazines that a department in the Army just ordered a bunch of the new Ruger .45s (like 5,000 of them or something.)
As for police departments. Most of the larger ones has some sort of public affairs office that you can call. Tell them you have a few questions about the equipment they use and an officer should be able to answer your questions.