That link shows the largest diameter you can get over a foot long is 3" diameter.
A six-foot piece of that would set you back $1000.
A yard-long barrel from that 3" brass would be $560.
yep. That website has several grades of metal available up to 6” diameter. Some are much cheaper than that page while others are even more pricey. I’m not sure what grade would be needed to make a cannon