Reason to fire a golf ball mortar.
1. Smoke...yep it will do that
2. Noise...yep it will do that although not real load
3. To watch the ball fly, yep you can do that.
4. To watch the ball the ball land (Or to see how close you can get to a target}.
Which can you do at 100 FFG grs.
1. Yep
2. Yep
3. For a while
4. You better be able too for safety sake.
In this case don't think max charge think how far do you want to shoot.
With a 1/2 chamber you can accurately lob a golf ball 30 yards with 24 Grs of FG. Start with that load and work up one grain at a time and see how far the ball will go for specific charge until you determine the maximum range you want to lob the golf ball. You don't need a chamber much longer the 3/4 inch for launch golf balls. Anything longer is just unneed and a waste of powder if used.
Switlik's chart for safe loads Cannons which you will find at the top of the board under safe loads says max for 1/2 inch cannon is 50 grains.
As far as wall thickness the rule of thumb is one caliber in all directions around the chamber. So if your chamber is 1/2 inch your walls should be at least 1/2 inch and that includes in the breech end.