A mention of the improvised Siege Mortars as used by Grant's troops during the Siege of Vicksburg, Mississippi.

"There were no Mortars with the besiegers, except what the navy had in the front of the city; but wooden ones were made by taking logs of the toughest wood (They used Gum Trees.HW), boring them out for six or twelve pounder shells, and binding them with strong iron bands. These answered as Coehorns, and shells were sucessfully thrown from them into the trenches of the enemy."

Confederate use of Howitzers as Mortars at Knoxville:

"All of our artillery, 34 guns, was posted in the most available positions to fire apon this Fort and enfilade the adjacent lines, except four Howitzers which were rigged as Mortars to drop shells behind the parapets and to search out spaces sheltered from direct fire. To accomplish this skids were prepared inclined at an angle of fourty-five degrees, one end resting on the ground and the other on a horizontal pole supported about six feet from the ground by forked posts. The axle of the Howitzer was run up on these skids, and leaving the trail on the ground between them, until the pieces had an elevation of sixty degrees. I had experimented with the arrangement in Virginia, and also at Chattanooga, and found it to work nicely and to give very fair Mortar practice. Of course the range was regulated by the charge of powder used."
BG Edward Porter Alexander, CSA