The mortar started out life as a 3 7/8 in. by 6 1/2 in. 4140 drop purchased from eBay for around $25. After around 15 hours of machining a mortar barrel was found inside (I wonder if the guy that sold it knew it was in there?). After another 20 hours it became a finished golf ball caliber coehorn.
At some point I will add some drawings and descriptions of construction techniques but this is what I have for now.
Rough turned
Finished barrel before bluing
Rough oak base
Finished base before linseed oil is applied
Everything together before bluing and linseed oil
The finished mortar
The mortar in action