The Lil' spud gun I own, well I must say its a bit fancy it fires caps as well (never knew that bit)the bore is of the eliptical type you will note.
We made up lemon guns looked a bit suss in a vege shop sizing lemons through a 75mm ring of PVC but it was effective punching through a hardi flex fence at @80 feet we found Aeroguard was good but the best was Aerostart would fire the lemons out of sight. If caught by the Police with them its a serious offence with one losing the real firearms owned.......so gave it up.