When there is a material inquiry as to the distance of the deceased from the defendant at the time of the homicide, as bearing upon the question whether deceased was near enough to strike the defendant, and whether the shooting was in self-defense, and it appears in evidence that no powder-marks were found upon the clothing or body of the deceased, it is proper to allow expert testimony for the prosecution as to the farthest distance at which clothing could or would be powder-marked with a rifle such as was used by defendant.