OK, I've been living with this secret long enough.
I got a 3" bore mortar, with a bunch of stuff with it, for free. It's a long story involving engine work, rebuilding an old cap and ball revolver, and some perfect timing.
Anyway, I had my gunsmith father check it out, and we "proof" fired it several times from a distance with 14 oz tin cans full of cement (actual weight: 2 lbs) and full powder loads (filled up the chamber with Fg black powder). We didn't see any problems.
I've been shooting it with half-full cans loaded nose first. They seem to shuttlecock early in flight and fly consistently. With a full charge of powder, and a can half full with 1 lb of cement in it, I get a range of about 280 yards at what I guesstimate is about a 60 degree barrel elevation.
Here is the mortar and accessories:
Here is just the mortar itself:
And here is a close-up of one of the projectiles I've made:
I still have some things to do, though: I have to make a worm (I've been using the kitchen tongs!), and a better linstock, and I *HAVE* to build a gunners quadrant so I know what elevation I am at.
