Remember, rifle bullets have more room to accelerate in the barrel. After the bullet leaves the barrel, speed is at it's peak and slows down from there (assuming you don't have a long barrel with a fast burning powder).
Compare the amount of powder and muzzle velocity of the rifle rounds to the 9mm, and you'll see the difference. It's possible that the shape of the round makes a difference, as well.
Compare the amount of powder and muzzle velocity of the rifle rounds to the 9mm, and you'll see the difference. It's possible that the shape of the round makes a difference, as well.