@RockRash, I sent a message with some info. https://www.millercannons.net
is who built my carraige. Steen Cannons, Trailrock Ordnance, Hanson Wheel, all make cannon stuff.
This is the correct barrel/ tube for the small carraige:
https://hernironworks.com/product/mountain-howitzer-full-scale-3-bore/
A link to order the plans and measurements.
https://www.dixiegunworks.com/index/page/product/product_id/526/category_id/543/category_chain/540,543/product_name/BK1570+12+Pdr.+Mountain+Howitzer+and+Pack+Carriage+Manual
For those that are getting the vapors, black powder muzzle loading cannons are legal and are not "destructive devices" federally. Not sure about different State rules, but in TN it's okay to use for rabbits and such.