I made a howitzer looking cannon out of 1.91 mortar tube. It takes about a dixie cup and a half of homemade bp and a golf ball wrapped in paper towel. I eventually blew the plug out (which was only stapled) at about 2.5 cups of powder. At all times I was behind a windshield at a safe distance. So is it safe?
BTW the plug blows at 0.55 and the barrel moving was my fault. The dowel that I brought broke on the first shot so I resorted to twigs.