Sometimes wedges were inserted to keep the shot or shell in place when the barrel was depressed.
From "ARTILLERIST'S MANUAL, COMPILED FROM VARIOUS SOURCES, THE AND ADAPTED TO THE SERVICE OF THE UNITED STATES"
http://www.civilwarartillery.com/books/gibbon.pdf
The 8-in. siege-howitzer, being loaded by hand, the fuze can be placed in - its proper position without the assistance of a sabot, which, when the piece was fired in the trenches over the heads of the working parties, would be dangerous to them. In loading this piece, as well as with the mortars, care should be taken that no powder remains along the bore; and in firing under small angles, or at a depression, the shell should be wedged up, to prevent its rolling out.
also
Ring or Grommet Wads , consist of a ring of rope -yarn about 0.7 in. thick, with two pieces of strong twine tied across it at right angles to each other. The size of the ring is the full diameter of the bore, in order that it may fit tight and stop the windage. They increase the accuracy of fire, and are preferred to the other wads when the object is merely to keep the shot in its place, as in firing at a depression. They stop the windage best when placed behind the ball. They may be attached with twine to the straps, or to the balls themselves, or may be inserted, like other wads, separate from the balls.