I want top thank you for making me terrified of drilling a vent hole. I have a Hern mortar barrel, and after reading this thread I rememberd that I have been letting three guys hunt out back for years that own a machine shop with an EDM machine. I cashed in the favor for a vent hole, and it came out really nice.
If you drill straight into the dimple cast into the Hern barrel you will be about .75 from the welded breech. My estimates are that it would take at least a 2 ounce charge to position the powder bag under the hole if drilled straight in.
I elected to have them angle it so that it came out at .40 from the breezh plate. The hole is .20 diameter. With such an angle, it will puncture a bag no matter how small the charge.
To set the EDM machin up, they clamped a bore sized (2.25) rod into the machine and positioned it to point .40 from the end of the rod. They then slid the barrel over the rod and checked to see if it was pointed at the cast in dimple. If it was not, they readjusted and tried again. After a while it both pointed at the dimple and also at the proper part of the 2.25 inch rod.
It worked out perfect.
So, I have to vented barrel, the hardware kit, about 100# of suitable dried white oak that i got from the lumber mill down the street for $40.
I even got a do-it mold and made myself a dozen nice lookin 2.11 inch 2# lead balls just to see what they would look like. ... quite a different experience from casting for my 44 mag - need to make a 3# ladle for the next casting session.
Just gotta find the time to sand, whittle, plane, cut, paint, and finish the concrete powder magazine out back and I'll be in business....
My goal is to be firing blanks by the 4th of July.