That can happen if the first round is a squib and the shooter continues to fire.
When I worked on commercial ranges, someone managed to fire five additional shots after the first squib in an older S&W K-frame revolver. The last shot split the barrel and you could see all six bullets stacked in there.
Okie John
Was shooting a match and the guy next to me stacked two bullets in the barrel of a S&W M27. It was the rapid fire portion of a PPC match and he was shooting very lightly loaded 148 gr. wadcutters through that heavy N frame. Somewhere in the fast string he fired a squib that stuck in the bore and followed up with another immediate shot that stuck that bullet behind the first squib load.
The second shot felt and sounded odd and he stopped firing the gun . He used a wooden dowel rod down the muzzle and tapped the two bullets back out of the forcing cone. There was no damage to the gun in this case.
