I bought the standard, M4 type 16 inch barrel from Del-ton with the standard sight. Well now I'd like to replace the sight with something like a flip up front sight. Obviously in order to do this one of the routes I can take is to buy a railed gas block. My problems comes in when buying the gas block, is I don't know what measurement to get. There are two, .936  and .750  I'm not sure which one to get.

I'm guessing you don't have a set of slide calipers or you wouldn't be asking this...

Anyway, you should be able to tell just by measuring it with a tape measure. .936 is basically 1", .750 is 3/4". If you can't get close enough by eyeballing it with a tape measure, wrap a piece of string around the barrel at the gas block, and measure the length of the string, divide the length of the string by Pi (3.14), and you'll end up with your diameter.

if you got a standard bbl. ie not lightweight or bull bbl., it should be .750 under the FSB.