New Mortar plan

[Freemax]

I picked up a piece of 1" brass rod (it's former use is unknown) at a local scrap yard to use as my first cannon/mortar piece.  I originally was going to use a 5/8" piece since that is all I had laying about.  I have attached a copy of the plan for it, based on the information on this board...this place is great. 
I plan on scaling it to fit the 1" OD of the mortar, but I have a few questions.  Due to the small scale and the fact that I am cheap, I planned on using .177 BB's for the bore and shot.  Now I am able to enlarge the bore, but what would be a safe caliber (this will be my first attempt) to "shoot for" so to speak.  I DO want to fire it, I mean what's the point right?  I figured that steel ball bearings or something like it would work.  Any and all help  (and comments on the drawing) would be greatly appreciated.
Thanks.
Cris

[GGaskill]

I think you would be OK to up the bore to 17/64" (for windage for a 1/4" ball bearing.)  I would make my chamber with a 1/4" ball end mill and not more than a 1/4" deep (a 17/64" ball end mill would be better but probably harder to find and more expensive) leaving at least 3/8" thickness at the bottom.  Get some really small diameter fuse and drill the vent to match what you get.  If you have it, an F drill (.257") would meet the windage formula and allow the use of the 1/4" ball end mill.

Your styling mixes that of the Coehorn (rings and reinforcement) with the Models of 1861 (center trunnions.)  I think you will find that center trunnions without welding will be a problem since you should cut pockets for them and solder them in place.  The pockets will weaken the barrel.  I think it would be better to go with the Coehorn style completely and use a rear trunnion.  You will have to cut a groove across the breech and stop the bore 3/8" in advance of the bottom of the groove but I think that would be a more satisfactory design.

[Freemax]

Your styling mixes that of the Coehorn (rings and reinforcement) with the Models of 1861 (center trunnions.)  I think you will find that center trunnions without welding will be a problem since you should cut pockets for them and solder them in place.  The pockets will weaken the barrel.  I think it would be better to go with the Coehorn style completely and use a rear trunnion.  You will have to cut a groove across the breech and stop the bore 3/8" in advance of the bottom of the groove but I think that would be a more satisfactory design.

I went with this style because I don't have access to any welding equipment.  I figured I could tap the trunnion holes and use either epoxy or some thread locker.  There was a detail posted in the forum that I knew I could make (see attached).  If I have to decrease the bore to accommodate the pockets, so be it.  I appreciate the reply, and I believe your posted designs were the catalyst for this one. 
Too bad I'm in Fl, I would really enjoy participating in one of your classes.