The .30-30 soft round was a type of ammunition that was planned for inclusion in Fallout: New Vegas. .30-.30 rounds were cut from Fallout: New Vegas due to it supporting few weapons on it's own and having too much overlap with .308.
The .30-30 soft round is the .30-30 Winchester/.30 Winchester Center Fire, originally created in 1895 for the Winchester Model 1894 lever-action hunting rifle. Given that the cowboy repeater is based on an older model of the rifle, the .30-30 may have been once planned as an ammunition type to be used by the rifle.
Only the icon for the ammunition is present in Fallout: New Vegas game files.