This thing shoots a potato around 350 mph and is a blast to shoot.
I would like to see a chronograph reading before I believe that your basic combustion can shoot at 513 feet per second, and 1/2 a mile is 2640 feet or 880 yards, with a combustion... I don't think so!
I like your cannon, but I think you're giving us a bit of a fish tale (If you know what I mean).
If you don't know what I mean let me make it clear "Liar, Liar, pants on fire!"