You also need to find the correct lenght of the gun (9 cm lenght) so you can work more freely inside it, without too many worries about this or that half milimeter, something that the late Dave Howitt tought me.
First you build the barrel out of Evergreen. One of the barrels was taken from the Airfix 5,5 inch gun after some shortening.
Then start to add more details as the back cover of the carriage and filling the carriage with other thin stripes of evergreen until you feel satisfied. You can avoid this trouble if you have the correct length of Evergreen rod, which I didn't.
Its wheel time: I used a pair of Britannia metal and two of Emhar´s plastic from its 18pdr.