I don't really understand. Grapeshot is single-shot, but machine guns fire many shots and target enemy troops. Are you saying it is impossible?
I'm explaining to you that it is more than possible, it can be done easily
The grapeshot is an example of the Cannon having two different firing options.
You can add any type of firing option you like. You can make the machine guns even have magazines, so that the unit can fire 10, 20 or 30 rounds continuously, but then has to reload after that amount, so he stops shooting and you can specify how long he takes to reload. This is done by adding a "ZALP" function to the units .MD file.
Try it your self, open up a units .MD file and add ZALP 5 to one of the lines, pack the .MD with the GSC File Utility and copy and paste the generated file in your American Conquest folder, start the game and select the modified unit, you will notice that it can shoot 5 consecutive times before having to reload.