I finally took my Coehorn out back and fired a few live rounds.
It is a 2.25 inch Hern Coehorn
Powder chardges were 1/2 ounce Cannon grade Goex.
I fired two 2# lead balls, and also a 5.25 inch long 2.125 diameter muffler pipe filled with concrete (about 1#)
I found both lead balls. They hit trees before stopping, and I found them at about 50-60 yards. I am thinking they would have landed at about 100 yards if the trees were not there.
The 1# concrete muffler pipe was not found. I heard it leave... then I heard nothing for a while... then I heard it splash down inthe trees... maybe 200-300 yards away would be my guess. My "range" is 500+ yards, but I could not see the impact, so I need to try to reduce the load for that projectile.
The vent hole is angled toward the breech (specifically to make small charges more possible), and enters the bore at .400 from the breech plug. The vent punch hits the breech about dead center when inserted into the vent hole.
When I fired the 2 pound balls, there was no issue puncturing the aluminum foil bag with a blunt (flat) vent punch.
When I fired the concrete pipe the blunt punch hit the pipe and would not enter the chamber. I have a pointy punch, which I found from firing blanks is more likely to cause a misfire. I tried it and was able to sneak it under the concrete pipe and fire the round. I think that with such small charges a couple of coorogated cardboard spacers may be needed to make sure I can get the vent punch to the powder.
I must admit, when I fired the 1# concrete filled muffler pipe the noise seemed way louder than when firing the 2# lead balls. I need to fire another one to get concurance on what happened.
For what it was worth, the muffler pipe was cut with a tubing cutter. The cutter tapered in the ends of the pipe. This no doubt made it easier for the concrete to stay put. From what I saw of the projectile leaving the bore there is little doubt that the concrete stayed in the pipe. I do wish I could have found it to look at it.
I painted all three projectiles flouresent blue to help find them, but only found the two lead balls.
The lead balls, while they hit trees, probably could be reloaded again with no issues.
This was all with 1/2 ounch charges of Cannon powder. From what I have been able to deduce the literature, 1.5 ounces of Cannon is probably a max load for 2-3 inch bore.
These things will really fly :-)