Ummmm.... no rifle is more powerful than any other. They are all empty tubes. The power and propulsion comes from the round (from the gunpowder within), not the firearm.
No one is arguing that a "rifle" sans cartridge is powerful. The rifle provides a chamber within which the self-contained cartridge can ignite its charge and allow the projectile to accelerate down a sufficiently long barrel and impart spin from rifling to stabilize the projectile during its flight. A longer barrel will impart more velocity.
A Remington Varmint Rifle in 308 Win with a 26" barrel produces more power than a Ruger Scout so chambered with its 16" barrel.
A cartridge without a rifle is so much brass, copper and powder -- pretty harmless without the instrument needed to activate it and direct it to its target.