He will have to change the materials to greatly stronger stuff if he wants more muzzle velocity.
Newton's third law of motion dictates the gun itself is subject to at least the amount of energy imparted to the projectile but there is far more energy and force going on within the gun than what is imparted to the projectile.
The current rail/gauss guns really want to split more than they want to fire a projectile.
Here is a picture to explain how much strength is required for even a small gun.
