You also need to remember that you will need to fill the entire barrel of the mortar to get the football to exit at any kind of speed. I'm not sure there would be enough volume in a single Cart to do that.
I'm working on a airsoft version of an ARMBRUST (German rocket launcher) that uses a paintball gun as the trigger section. I'm reducing the barrel size (.68 to .375) to allow a metal tube to fit down the hollow inside of some foam and rubber rockets I have. It will load from the front. Paintball gun will be inside the main tube section (4 inch plastic pipe as it doesn't need to hold pressure) Air bottle will be stored in the back section of the fake launch tube.
Nothing says loving, like a head shot!