Was going to take 1899 to the range today, tried a 38 special in it, wouldn't fully seat. So located some short colt rounds to test functionality at range. Later found some 125gr in special brass, still will not seat , would have to crimp ahead of crimp grove.
It is not the bullet, an empty 38 special case will not seat, 38 special brass .17 too long. So I am guessing 38 Long Colt. Barrel stamped S&W 38 Mil.
The military model was made in 38 Long Colt.
Both civilian 1899 I have will shoot 38 Special.
Brian