There is no guarantee that any cannon or mortar will not fail. You can improve the chances that it will not fail by following recommended safety standards. We have posted at the top of the forum a Sticky called safe loads that will direct you to the safety guidelines
The Florida mortar and any mortar built like it, has a serious design flaw. The front of the powder chamber is welded to the bottom of the tank.
In this design the cannon is the powder chamber. The tank is simply a device to hold the ball on the muzzle.
When the cannon fires the muzzle blast is deflected by the base of the ball, and high pressure heated gas impacts directly on the weld. Just like with welded seam tube the weld can become contaminated and fail. Probably won't happens soon, but will be a surprise when it does.
It would be better if the chamber were attached to a trunnion and put inside the tank---look better also. With the chamber attached to a trunnion should the weld between tank and chamber fail the trunnion will contain the chamber.
You chamber-barrel is 4.5 inches in diameter. Safe recommend powder chamber diameter is 1.5 inches and maximum load should be 500 grains of Cannon grade. Industry standard for a proof load 125% of PMax-Maximum peak pressure. So a proof load for your cannon would be 125% of the pressure of a 500 grain load with bowling ball generates. Not 3 or 4 ounces and for sure not a pound.
A cannon of the dimension above that has been subjected to such massive over loads have a very real chance of suffering structural damage and should be consider unsafe for further use.
