Is there a website for finding info on firearms by entering the make and serial number?

None that will give you all the information available. Antique and Collectible Firearms and Militaria Headquarters has year of manufacture data for some. In many cases, even this information is not available as many manufacturers records no longer exist, and many firearms never had serial numbers as they were not required by law in the US until 1968.

AnswerNot one website. But companies that are still around do. I know for a fact that the official websites of Remington, Browning, and Ruger have archives that have year of manufacture etc. of guns made over 100 years ago, if you have the SN. It takes some link clicking and searching to find the correct web page sometimes. But they do have it. AnswerIf the manufacturer is still around, a quick phone call to them can usually determine this information. Companies like Colt and Winchester will provide historical information on 1st and 2nd generation colt single-action army revolvers. Any other firearms can be done via email. You can contact their Archive Department.

The Remington Society of America is a dedicated team of enthusiasts that research all sorts of info on Remington Firearms.

Proofhouse.com has Browning, Colt and Winchester models/sn's

There are a fairly large number of on-line firearm forums. Most of these have sub-forums that will allow the asking of questions about specifics of your firearm, that will be answered by quite a few knowledgable people. See links below for a few of these sites.