1" Bore 24 Pounder Coehorn Mortar

I had a hunk of 1-15/16" 1018 round bar left over from building a 1" bore mountain howitzer a while ago, so I started going through my folder of cannon plans to see what would fit. A Coehorn mortar seemed to be the best use of the material. I drilled a .48" diameter hole through the center near one end to eventually take the .5" trunnion.

Next I turned down the cylinder to the maximum outside diameter, and faced off the two ends to the right length relative to the hole for the trunnion.

Next I drilled the 1" bore, and a .45" powder chamber. Since the entire thing isn't cast, the trunnion cuts into the meat of the tube. To keep the 1:1 ratio I had to drill a pretty short powder chamber. Its maximum capacity is 16 grains.

Next I turned the outside profile...

...and welded on the trunnion. It looks worse than it is because it's a bit out of focus. I cleaned the joint up with a dremel and files, then smoothed out the transition from tube to trunnion with Bondo. I'm not sure why I went to so much work because most the joint will be inside the mortar bed.

Next I made the mortar bed. It's just made out of poplar, but I really don't intend to shoot this piece much. I cut the ends on a 45º to get rid of most of the wood, then used a router table and a round over bit.

Then I inlet the tube into the bed. I drilled out the rectangular section with a 7/16" Forstner bit and then cleaned the mortise up with a few sharp chisels. Then I thought I'd just wing the curved section because it honestly it couldn't be that hard. After a few minutes of that I realized it really was that hard so I got a candle to coat the tube in soot. After placing the tube into the bed, I shaved off the resulting black spots with a gouge. About 2 hours and 3 singed fingers later, I had it at 45º.

Using my grade AA kitchen counter top and super precise measuring instruments to insure it's 45º :D

On to the hardware. I bought some 8-32 square nuts off McMaster-Carr and used my scrap pile of reject CSS Virginia armor plate to make the trunnion straps and handles. I've heard lots of people complain about how time consuming the handles can be. Naturally I didn't think they'd be all that hard to do, but I quickly found out they were right! I didn't keep track of time, but I'm almost positive I spent more time on the handles than I did the tube. I thought about only making two sets and then just taking pictures of one side of the mortar.

All the parts. The 8-32 threaded rods are just cut off bolts epoxied into the mortar bed.

Here's the size of the mortar compared to a powder can.

Here's a video of the build with a clip of the first shot toward the end. The charge was 16 grains FFg and an ounce of birdshot over a wad.