[Travis McGee]

I rather think the thickness a bit inadequate.

So do I, but I'm only guessing. In Charleston SC along "The Battery," the promenade along the harbor facing Fort Sumter, they have some old siege mortars. Those bad boys have a similar bore, but the wall thicknesses are about equal to the bore, or about 8" wall thickness! Of course, they tossed exploding iron cannonballs about a mile or so!

[eno_]

They also used powder several pounds at a time, and projectiles 10X-50X more massive than a bowling ball.

[archy]

I rather think the thickness a bit inadequate.

So do I, but I'm only guessing.

Likewise, the bowling-ball mortars made from cutdown oxygen welding gas cylinders are thinwall, though strong.

But then too, look at the aluminum barrel of the M79 and M203 grenade launcher, and the thin-walled cartridge case of the 40mm grenades they use. Using a high pressure chamber built into the base of the projectile, which then directs the high pressures generated against the much wider base of the projectile, the resulting high/low combination has proven safe and reliable. Something similar may be in use with at least some of the bowling ball launchers.