Help with mortar

[Smokin Joe]

A buddy of mine just received a beer-can sized mortar with 3/4" wall thickness. I checked the powder chamber volume, and it'll hold between 380 and 400 grains Fg.
 Will 200 grains or less work in this thing safely? I'm wondering about the air space in the chamber if used with less than full charges.

[Double D]

The rule of thumb for maximum charge is  2 oz of FG per inch of bore diameter.  For a mortar the diameter of the powder chamber is considered the bore diameter  for computing powder charges.

Safe wall thickness at the breech would be a wall equal in thickness to the diameter of the bore in all directions around the breech.

In mortars air space is not an issue.  The chamber is the bore and is smaller than the area containing the projectile.  The portion of the barrel that holds the projectile is an expansion chamber.  Cannon and mortar projectiles also need windage which makes them easier to load with fouled bores and rusty out of round shot.

The rule of thumb for windage is 1/40 of bore diameter. Simplified divided the beer can diameter by 39 and multiply by 40 and that should be the correct bore diameter for the mortar.

[Cat Whisperer]

Agree.  The air space is the same each time.  A round that fits with windage won't cause any undo pressure even with a bit air space.

We'd love to see some pictures!