What is the value of a Daisy bb gun model 799.19051?

That depends, if it is an original from the late 1930's or early 1940's, then it could be worth hundreds, maybe thousands. It also depends on if it is still in it's original packaging because that will play a large part into it's sale price. You might want to check on collector's websites for more information and potential prices.

You may also check with Daisy. They offer a free service to tell you more about your old Daisy Antique. See the link below.