Shell holders for firearms have a body including a plurality of bores, a plurality of elastomeric elements, each associated with a different selected one of the bores, each elastomeric element having a protruding portion that protrudes a selected distance into the bore, a plurality of movable adjustment elements, each connected to the body and each operably engaged to a different selected one of the elastomeric elements, wherein the selected distance the protruding portion of the elastomeric element protrudes into the bore is adjustable based on an adjustment position of each adjustment element, and the selected distance is independently adjustable for each elastomeric element. The shell holder may be for a shotgun. The movable adjustment elements may include a plurality of jaws movable with respect to the body. There may be a gap defined between the body and each jaw. The elastomeric elements may be received within the gaps.